PARIS -- Seeking to move beyond Britain's blindsiding rejection of military intervention in the Syrian conflict, the Obama administration received strong support from France on Friday and released a detailed intelligence summary to buttress its contention that the Syrian government used chemical munitions in an Aug. 21 attack, asserting for the first time that it had killed 1,429 people, nearly a third of them children.
The intelligence summary, cited by Secretary of State John Kerry in a televised briefing at the State Department, contained by far the most specific contentions the administration has publicly made in arguing that the Syrian government had crossed a threshhold of intolerable behavior in its effort to defeat insurgents in the civil war, justifying an international military response.
The administration had previously not asserted such a precise death toll in the Aug. 21 attack, which rights activists and medical aid workers say left hundreds asphyxiated in what appeared to be the most egregious mass killing in the conflict, now in its third year.
A summary of the intelligence assessment said its conclusions were based on "human, signals and geospatial intelligence as well as a significant body of open-source reporting."
Among the findings, the summary said, was intelligence that Syrian chemical weapons personnel had plotted the attack for three days in advance, partly out of frustration that the use of conventional weapons in Ghouta, one of the Syrian towns hit last week, had failed to dislodge insurgents.
The intelligence found "activities that we assess were associated with preparations for a chemical weapons attack" in the three days before the weapons were unleashed, the summary said.
The administration has moved missile-armed warships close to the Syrian coast in preparations for a possible strike, even as United Nations chemical weapons inspectors on the ground in Syria were still conducting research to ascertain whether chemical munitions were in fact used in the attack in Ghouta, a suburb of Damascus, the Syrian capital.
A spokesman for Ban Ki-moon, the United Nations secretary general, said he had given the five permanent members of the Security Council "an overview" of the inspector mission's work on Friday and that the inspectors intended to leave Syria on Saturday with their samples and other information.
The spokesman, Martin Nesirky, also said Mr. Ban would confer on Saturday with Angela Kane, his top disarmament official, who was returning from Syria. He declined to specify what -- if anything -- the inspectors had concluded so far.
He also declined to specify when the inspectors might complete their report. "We're not giving a timeline on that except to say they are doing this as swiftly as possible," he told reporters at the United Nations.
The Obama administration has made clear, however, that it has already concluded that chemical munitions were used in Syria and that the government of President Bashar al-Assad was culpable. Prime Minister David Cameron of Britain asserted the same position, and the British had been expected to join the Americans in a possible military strike aimed at Mr. Assad's forces. But in a surprise vote on Thursday, British lawmakers rejected Mr. Cameron's plan, opposing what they called an ill-advised rush to military action that recalled Britain's alliance with America in the Iraq war 10 years ago.
President François Hollande of France, however, on Friday offered strong support for international military action against the Syrian government.
The Aug. 21 chemical attack "must not go unpunished," Mr. Hollande said in an interview with Le Monde, the French daily newspaper. "Otherwise, it would be taking the risk of an escalation that would normalize the use of these weapons and threaten other countries."
A military strike against government targets would have a "deterrence value" and push President Assad toward a negotiated "political solution" to the conflict, said Mr. Hollande, referring to France's explicitly stated goal.
Mr. Assad has denied responsibility for the use of chemical weapons. Backed by allies Iran and Russia, he has said that if chemical munitions were deployed, the insurgents trying to topple him must have used them.
Although Mr. Hollande has presented no specific evidence linking the Syrian government to the attacks, he has spoken confidently of its culpability. Parliamentary approval is not required for French military action, and Mr. Hollande has said his government is "prepared to punish" those responsible.
"France possesses a body of evidence that goes in the sense of the regime's responsibility" for the chemical attacks near Damascus, Mr. Hollande said. The use of chemical weapons there is an "established fact," he said, and "it is known that the opposition possesses none of these weapons."
The government of Turkey, an outspoken opponent of Mr. Assad, added its voice on Friday to those who have concluded that Mr. Assad's forces used the weapons and who assert that they have proof. Ahmet Davutoglu, Turkey's foreign minister, said in a televised statement that Turkish intelligence sources had what he called "healthy information" that implicated the Syrian government.
"The regime responsibility is undeniable when launching vehicles, angles between launching locations and targeted regions, traces are considered," Mr. Davutoglu said, without further elaborating on the precise origins of such information.
By contrast, Iranian officials, who frequently remind the world that they were targeted with Iraqi chemical weapons during the Iran-Iraq war of the 1980s, said Mr. Assad's government had assured them that it had never used such munitions.
"Iran, as a main victim of use of chemical weapons, is against any kind of usage of this inhuman warfare," Iran's ambassador to the United Nations, Mohammad Khazaee, said in an e-mailed statement to The New York Times. He urged other countries to let the United Nations inspectors in Syria complete their work, and he indirectly criticized the United States and France for moving missile-armed warships closer to the Syrian coast.
"We are against any foreign military intervention, which will damage the efforts aimed at finding a political solution for the conflict in Syria," Mr. Khazaee said. "We believe that missiles have never been peace messengers in the Middle East and the rest of the world."
France's determination contrasted sharply with the go-slowly approach of Germany. "We are pressing for the United Nations Security Council to reach a common position and for the U.N. inspectors to conclude their work as soon as possible," the German foreign minister, Guido Westerwelle, said in comments reported by a regional newspaper, The Neue Osnabrücker Zeitung. Referring to German participation in a military strike, he said, "Such participation was not requested of us, and neither are we contemplating it."
Chancellor Angela Merkel's government and the opposition have been cautious on Syria, anxious in a war-averse country not to have a military conflict become part of the campaign for elections on Sept. 22. Ms. Merkel's main rivals, the Social Democrats, have urged President Obama to delay any military action at least until after he meets fellow world leaders at Group of 20 summit meeting in St. Petersburg, Russia, late next week. Ms. Merkel, while condemning the chemical weapons attack in strong terms, has avoided any comment on suggestions from the Social Democrats that she mediate between the Russians and Americans on Syria.
In a telephone call Thursday with Mr. Hollande, Ms. Merkel said that while Germany declined to take part in a military action, she believed that the chemical weapons attack should not go unpunished.
Mr. Hollande's interview appeared just a day after Mr. Cameron was handed a stinging rebuke in the House of Commons, where Parliament rejected British military participation in any strike on the Syrian government.
British legislators rejected a motion urging an international response to the chemical weapons attack by a vote of 285 to 272, reflecting concerns that there was insufficient evidence that the attack had been carried out by forces loyal to Mr. Assad. Lawmakers were also worried about the strategy behind the call for limited strikes, which they feared could escalate the conflict and strengthen opposition forces aligned with Al Qaeda.
The United States will continue trying to build an international coalition, Defense Secretary Chuck Hagel said Friday in Manila, the Philippine capital.
"Our approach is to continue to find an international coalition that will act together," Mr. Hagel said. "And I think you're seeing a number of countries state, publicly state, their position on the use of chemical weapons."
Mr. Cameron addressed the parliamentary defeat on Friday, telling journalists in London: "We will continue to take a case to the United Nations. We will continue to work in all the organizations we are members of -- whether the E.U., or NATO, or the G-8 or the G-20 -- to condemn what's happened in Syria.
"It's important we uphold the international taboo on the use of chemical weapons," he continued. "But one thing that was proposed, the potential -- only after another vote -- involvement of the British military in any action, that won't be happening."
Mr. Obama has no hope of obtaining a mandate for a military strike in the United Nations Security Council. Russia, Syria's longtime backer, has long opposed military intervention of any sort, and China, which has urged that no decision be made until the results of the investigation by United Nations inspectors are revealed, has continued to push for more diplomacy.
On Friday, Yuri V. Ushakov, the top foreign policy aide to President Vladimir V. Putin of Russia, said Russia was "actively working to avoid a forceful intervention in Syria," the Interfax news service reported.
"We would not like the situation to approach one when one government, or group of governments themselves, issue accusations, judge them and carry out their own personal sentence," he said.
Mr. Ushakov said the United States had not shared intelligence showing that Mr. Assad was behind last week's chemical weapons attack, and "we do not believe it."
Aleksei K. Pushkov, chairman of the Russian Parliament's foreign affairs committee, described the British Parliament's rejection of military action as evidence of a "deepening schism" in the West.
"The refusal of Great Britain to support aggression against Syria is the strongest strike against the positions of the supporters of war, both for NATO and for the U.S.A.," Mr. Pushkov said in a Twitter post. "The schism is getting deeper."
Mr. Pushkov wrote earlier that Mr. Obama wanted to strike Syria this week, before his departure for the Group of 20 summit meeting in St. Petersburg, Russia, next week.
George Osborne, chancellor of the Exchequer and a senior politician in Mr. Cameron's Conservative Party, told the BBC on Friday that Parliament's rejection of military action would bring "a national soul-searching about our role in the world and whether Britain wants to play a big part in upholding the international system."
He was optimistic, however, that Britain's "special relationship" with the United States would survive. "There's a bit of hyperbole on this in the last 24 hours," he said. "The relationship with the United States is a very old one, very deep and operates on many layers."
The British Foreign Office on Friday warned its citizens against "all but essential travel" to Lebanon, particularly areas close to the border with Syria, citing "the recent upsurge in violence and wider regional tensions."
David Jolly and Scott Sayare reported from Paris, and Rick Gladstone from New York. Reporting was contributed by Alan Cowell from London, Andrew Roth from Moscow, Alison Smale and Victor Homola from Berlin, and Sebnem Arsu from Istanbul.
This article originally appeared in The New York Times.