LONDON -- Prospects for a Western-led military strike on Syria increased on Tuesday as the American defense secretary said United States forces were ready for any contingency, the British and French leaders spoke with an aggressive new tone and the Arab League joined in accusing the Syrian government of a mass killing last week with a chemical munitions attack.
The developments came as United Nations weapons inspectors in Syria postponed a second visit to suspected attack sites on the outskirts of the capital, Damascus, after having failed to secure assurances of their safety, the United Nations and Syrian officials said.
Even without the evidence that the inspectors are collecting, the United States and other Western powers have concluded that the Aug. 21 attack, which killed hundreds of people, was caused by banned chemical munitions and that President Bashar al-Assad's forces were responsible, crossing a threshold that required a forceful response.
Chuck Hagel, the United States secretary of defense, said in an interview with the BBC that American forces had "moved assets in place to be able to fulfill and comply with whatever option the president wishes to take." Asked how soon these forces could be ready, Mr. Hagel said, "We are ready to go."
Mr. Hagel would not specify the type of action envisioned, but Obama administration officials have suggested that any military response would be limited -- cruise missiles launched from American warships in the Mediterranean that would strike specific Syrian military targets, for example -- and not a sustained bombing campaign intended to topple Mr. Assad, who is seeking to defeat an insurgency well into its third year.
The White House press secretary, Jay Carney, told reporters in Washington there was no doubt Mr. Assad's side was responsible and that the only debate was over the proper response. "We believe that a careful review of the facts leads to the conclusion that the regime was behind this," Mr. Carney said.
In coming days, Mr. Carney said, the White House would provide further evidence, drawn from American intelligence, on the Syrian government's culpability. But he said President Obama had still not made a decision on military action.
In Britain, Prime Minister David Cameron, who cut short his vacation, said Parliament would be recalled early from its summer recess to deal with the Syrian crisis. British media said fighter aircraft had been dispatched to Cyprus, where Britain maintains an air base that could be used as a launching area against Syria, 100 miles away.
Mr. Cameron told the BBC that Mr. Assad's side was responsible for a "massive use" of chemical weapons, called it "morally indefensible" and said that the world could "not stand idly by."
President François Hollande of France said that "everything leads us to believe that it is the Syrian regime that committed this abject act" and said France would now seek to establish "the most appropriate retaliation."
"The chemical massacre in Damascus cannot go without a response," Mr. Hollande said in an address to an annual meeting of French ambassadors at the presidential palace in Paris. "France is prepared to punish those who took the vile decision to gas innocent people." He gave no specifics, however.
Previously, French officials had said firmly that France would not act militarily without an authorizing resolution from the United Nations Security Council, which seems doubtful because of Russian and Chinese opposition.
Mr. Cameron said no decision had been made about military action, but it would have to be proportionate and legal. He, too, did not define what the legal basis would be for military intervention. He said that any military response would be limited to the effort to "deter and degrade the further use of chemical weapons."
The British deputy prime minister, Nick Clegg, told reporters that "we're not considering an open-ended military intervention with boots on the ground like we saw in Iraq," which his Liberal Democrats had criticized, or "regime change." He added: "This is about taking proportionate, legal and carefully circumscribed steps" so the world understands "that we will not stand idly by when chemical weapons are used in complete breach of international law."
Foreign Minister Guido Westerwelle of Germany, which was criticized by Western allies for its refusal to take part in the NATO bombing campaign in Libya two years ago, has been sounding more aggressive about Syria. On Tuesday, Mr. Westerwelle called the Aug. 21 attack in Syria a "crime against civilization."
If the use of chemical weapons is confirmed, he said, "then the international community must act. Germany will be among those who consider that consequences are the proper thing." In this, he said, Germany is in constant, close contact with its allies "and above all the United Nations."
In Cairo, the Arab League, which has long antagonized Mr. Assad by supporting the Syrian opposition, held an emergency meeting and blamed Mr. Assad's government. In a communiqué, the group said it held "the Syrian regime fully responsible for this heinous crime and demands that all those involved in this abominable crime be subjected to fair international trials like other war criminals." But the group stopped short of endorsing an international military response.
Mr. Assad and his subordinates have denied responsibility for the attack and have asserted that insurgents fighting to topple him carried it out. Russia and Iran, Mr. Assad's principal supporters in the conflict, have backed his version of events and warned against any Western military intervention.
On the ground in Syria, United Nations inspectors, who came under sniper fire on Monday before a visit to one location, had been set "to continue their investigation in a different site" on Tuesday, the United Nations said in a statement. But after the attack on Monday, "a comprehensive assessment determined that the visit should be postponed by one day in order to improve preparedness and safety for the team." The statement said the inspectors had not received "confirmation of access."
Syria's foreign minister, Walid al-Moallem, said that the inspectors' trip had been delayed by one day because of disputes among the rebel groups. The minister said the insurgents could not agree on issues related to guaranteeing the inspectors' safety. He gave no further details.
The recall of British lawmakers was apparently designed to secure parliamentary support for action and to head off complaints they had been sidelined.
Earlier, Mr. Cameron's spokesman said, "We are continuing to discuss with our international partners what the right response should be, but, as part of this, we are making contingency plans for the armed forces."
Mr. Cameron's support of the United States recalled earlier moments of crisis under his government and that of his predecessor, Tony Blair, when Britain projected itself as playing a decisive role in far-flung crises at America's side, even though the United States wields far greater military clout.
Britain seems anxious to maintain the impetus of efforts to devise a tough response to the attack in Syria.
The Press Association of Britain quoted officials as indicating that a decision on the nature of any military response could be taken before the United Nations inspectors report on their findings.
In Washington, using some of the most aggressive language used yet by the Obama administration, Secretary of State John Kerry on Monday accused the Syrian government of the "indiscriminate slaughter of civilians" and of cynical efforts to cover up its responsibility for a "cowardly crime."
The issue has deepened the divide between the United States and Russia, Syria's main international ally and sponsor.
Moscow warned on Tuesday that a military intervention in Syria could have "catastrophic consequences" for the region and called on the international community to show "prudence" over the crisis.
"Attempts to bypass the Security Council, once again to create artificial groundless excuses for a military intervention in the region, are fraught with new suffering in Syria and catastrophic consequences for other countries of the Middle East and North Africa," the Foreign Ministry in Moscow said.
Alan Cowell and Steven Erlanger reported from London, and Rick Gladstone from New York. Reporting was contributed by Mark Landler from Washington, Scott Sayare from Paris, Alison Smale from Berlin and David D. Kirkpatrick from Cairo.
This article originally appeared in The New York Times.