STRELNA, Russia -- President Obama ran into an impasse on Friday in his bid to rally international backing for a military strike on Syria as world leaders wrapped up a summit meeting here remaining deeply divided over the right response to what the Americans have called the deadliest nerve gas attack in decades.
After a dinner debate that lasted into the early morning hours of Friday, Mr. Obama emerged with a few supporters but no consensus, as other leaders urged him not to attack without United Nations permission, which is not forthcoming. Instead, the president had to resign himself to generalized statements of concern over the use of chemical weapons.
Even France, which has offered the strongest support to Mr. Obama of the European allies, on Friday said that it would not strike Syria as part of a coalition until the United Nations completes its work on assessing the suspected use of chemical weapons in Syria. Ban Ki-moon, the United Nations secretary general, has declined to specify when the results will be known.
The failure to forge a stronger coalition here in the face of opposition from the Russian host, President Vladimir V. Putin, raised the risks even further for Mr. Obama as he headed home to lobby Congress to give him the backing his international peers would not. It also left Mr. Obama in the awkward position of defending his right to take action largely alone if necessary after campaigning against what he portrayed as the unilateralist foreign policy of his predecessor, George W. Bush.
Mr. Obama acknowledged that he had a "hard sell" with Congress and announced that he would deliver a televised address to the nation Tuesday evening from the White House.
"Failing to respond to this breach of this international norm would send a signal to rogue nations, authoritarian regimes and terrorist organizations that they can use W.M.D. and not pay a consequence," he said at a news conference, using the initials for weapons of mass destruction. "And that's not a world we want to live in."
But much of the world, at least as represented at the Group of 20 meeting here in this St. Petersburg suburb, did not favor Mr. Obama's proposed course of action. Mr. Putin said a majority of the leaders joined him in opposing a military strike independent of United Nations approval, including those from Argentina, Brazil, China, Germany, India, Indonesia, Italy and South Africa.
Citing remarks by Jacob Zuma, the South African president, Mr. Putin said: " 'Small countries in today's world in general are feeling increasingly vulnerable and unprotected. There is an impression any superpower at any moment at its discretion may use force.' And he's right."
The only countries that supported Mr. Obama's plan, the Russian leader said, were Canada, France, Saudi Arabia and Turkey, all nations that were on Mr. Obama's side when he arrived here on Thursday.
Trying to counter the impression of isolation, the White House arranged for a joint statement including those allies as well as Australia, Britain, Italy, Japan, Spain and South Korea condemning the Aug. 21 chemical weapons attack in the suburbs of the Syrian capital, Damascus, which according to American intelligence agencies, killed more than 1,400 people.
"We call for a strong international response to this grave violation of the world's rules and conscience that will send a clear message that this kind of atrocity can never be repeated," the statement said. "Those who perpetrated these crimes must be held accountable." Still, the statement did not explicitly endorse military action.
But Mr. Putin pointed to a statement issued by Pope Francis on Thursday opposing military strikes and cited polls showing that the citizens of most countries, including the United States, also did not favor an American-led strike. "I can assure you -- and the latest polls say this as well -- the overwhelming majority of the populations in these countries is on our side," he said, his voice rising combatively.
Even as Mr. Putin ardently argued against an American-led intervention, Russia's Navy continued preparations in the event of an attack. It has already dispatched at least four warships to the Mediterranean Sea, including three that passed through the Bosporus on Thursday, two landing ships and a destroyer.
Russian news agencies on Friday reported that additional ships would join the armada, but not until later in September. Mr. Putin's chief of staff, Sergei B. Ivanov, told reporters that the landing vessels were being sent in case it was necessary to evacuate Russian citizens from Syria.
Russia's deputy defense minister, Anatoly I. Antonov, said on Thursday that while the ships were "an attempt to deter other forces that are ready to launch military actions in the region," they did not intend to intervene against American and other NATO warships that have also assembled in the region. "We don't intend, either directly or indirectly, to take part in a possible regional conflict," he said, according to the Web site of the Ministry of Defense.
Speaking with reporters as he was about to end his three-day overseas trip, Mr. Obama repeatedly refused to say whether he would abide by the Congressional vote he asked for authorizing the use of force against Syria if lawmakers say no.
"You're not getting any direct response," he said. But Antony Blinken, his principal deputy national security adviser, told NPR that while the president maintains that he has the authority to act regardless of Congress, "it's neither his desire nor his intention to use that authority absent Congress backing him."
The Syria dispute came to dominate the G-20 meeting and underscored the difficulty Mr. Obama has faced with Mr. Putin in recent months. After Russia gave temporary asylum to Edward J. Snowden, the National Security Agency contractor who disclosed secret American surveillance programs, Mr. Obama canceled a separate one-on-one meeting with Mr. Putin in Moscow.
But the two ended up talking on the sideline of the group session on Friday, mainly about their disagreement over Syria. Mr. Obama said Mr. Snowden's case did not really come up. "It was a candid and constructive conversation, which characterizes my relationship with him," Mr. Obama said.
For his part, Mr. Putin said the two leaders agreed to disagree during a friendly encounter on Thursday that lasted more than 20 minutes.
"We hear each other and understand the arguments," he said. "We simply don't agree with them. I don't agree with his arguments and he doesn't agree with mine, but we hear and try to analyze."
He added that they did agree that Syria ultimately needed a political settlement, and delegated the question to Russia's foreign minister, Sergey V. Lavrov, and Secretary of State John Kerry. In May, the two officials announced an effort to begin negotiations for a settlement in Syria, to be held in Geneva, but that effort has since stalled and now seems further away than ever.
The president said he appreciated that Mr. Putin had allowed a full airing of views about Syria at the dinner on Thursday. By several accounts, it was a vigorous discussion in which Mr. Obama and Mr. Putin in effect were competing for support. Mr. Obama emerged having changed no one's mind about military force, but most of the leaders at least agreed with his assessment that the government of Bashar al-Assad, the Syrian president, was responsible for the attack, something Mr. Putin has dismissed as "utter nonsense."
"I've been encouraged by my discussions with my fellow leaders this week," Mr. Obama said Friday. "There is a growing recognition that the world cannot stand idly by."
But he acknowledged the deep reservations over the use of force and said he reminded the leaders at the dinner that he had opposed Mr. Bush's invasion of Iraq in 2003. "I was elected to end wars, not start them," Mr. Obama said he told them. "I'm not itching for military action."
In addition to the talk with Mr. Putin, Mr. Obama held more formal meetings with the leaders of Brazil, China, France, Japan and Mexico. His session with President François Hollande of France was his one meeting with an unalloyed supporter of military action against Syria. "Doing nothing would mean impunity," Mr. Hollande told reporters, "and there would be a risk of repeating, so we must take responsibility."
Mr. Hollande has called forcefully for intervention and spoken with certitude about Mr. Assad's responsibility for the chemical attack. France has no intention of intervening without an international coalition, however, and Mr. Hollande has been obliged to await the Congressional vote before taking action of any kind. On Friday, he announced that France would also await the findings of United Nations weapons inspectors who visited the site of the Aug. 21 attack.
"We're now going to wait for the decision by Congress," Mr. Hollande said, "then the inspectors' report. And in light of these elements, I will, here again, have to take decisions."
Mr. Hollande offered no explanation for his decision to await the United Nations findings, but French lawmakers have in recent days increasingly called for him to do so.
The Chinese president, Xi Jinping, tried to dissuade Mr. Obama from the use of force during their talk on Friday, said Benjamin J. Rhodes, deputy national security adviser. "We've obviously had a difference with China on this issue," he said.
President Obama's conversations with President Dilma Rousseff of Brazil and President Enrique Peña Nieto of Mexico were intended to smooth over consternation in both countries about reports, based on Mr. Snowden's leaks, that the National Security Agency eavesdropped on both leaders' telephone calls, e-mails and text messages.
Ms. Rousseff told reporters afterward that Mr. Obama promised an investigation, but she held out the possibility that she might cancel a trip to Washington scheduled for next month. Mr. Peña Nieto told BBC that he also got a promise of an inquiry into the allegations and that Mr. Obama committed "to impose corresponding sanctions" if they were true.
While White House officials said that tension did not influence the Syria debate, neither leader backed military action.
Mr. Obama also made a point before leaving town of meeting with nine Russian activists to show support for groups and individuals who have come under pressure from Mr. Putin's government. Among them was a leader from a gay rights group, raising an issue that has grown especially sensitive in Europe and the United States since Russia outlawed pro-gay "propaganda" this summer.
But Mr. Obama had no critical words for Mr. Putin or his government during his comments in front of news cameras, instead focusing on his own history as a community organizer and offering general statements about the value of free press, independent opposition and civil society.
Mr. Obama was scheduled to land back in Washington on Friday night as he braced for what advisers consider one of the most critical Congressional debates of his presidency.
"I knew this was going to be a heavy lift," he said. "I was under no illusions when I embarked on this path. But I think it's the right thing to do. I think it's good for our democracy. We will be more effective if we are unified going forward."
Scott Sayare contributed reporting from Paris.
This article originally appeared in The New York Times.