As momentum toward an imminent Western military strike on Syria appeared to slow, United Nations inspectors headed to the outskirts of Damascus for a third day on Thursday, seeking evidence of chemical attacks while the British authorities took the unusual step of publishing an intelligence assessment blaming the Syrian government for the deadly onslaught.

Ban Ki-moon, the United Nations’ secretary general, told reporters in Vienna that the inspectors would complete their work on Friday and report to him on Saturday about their inquiry into attacks last week that left hundreds dead.

The inspectors headed in a six-car convoy on Thursday toward the site of attacks in Ghouta, an eastern suburb of Damascus, to collect evidence and samples, activists said, and were focusing on the Zamalka area. Their mandate is not to apportion blame for firing chemical weapons but to establish whether they were used.

In London, government enthusiasm for a rapid retaliatory strike against Syrian government targets seemed to evaporate late Wednesday when British leaders, facing dissent among lawmakers, signaled that they would await the inspectors’ findings. Prime Minister David Cameron’s government, aware of the sensitivities created by the Iraq war, said unexpectedly that a separate vote would be required later, possibly next week, to authorize military action.Mr. Cameron bowed to pressure from the opposition Labour Party and to some within his own coalition who want to wait for the weapons inspectors and for the United Nations Security Council to make one more effort to give a more solid legal backing to military action against the government of President Bashar al-Assad of Syria. Mr. Cameron’s government submitted a motion for debate Thursday in Parliament.

Mr. Cameron, who heads a coalition government, is facing political difficulties from legislators mindful of events in 2003, when assurances from Prime Minister Tony Blair and President George W. Bush that Saddam Hussein had weapons of mass destruction proved to be inaccurate and a false pretext for war.

Ahead of the planned debate Thursday, the British government published legal advice which argued that, under international law, targeted military action in Syria was justified on humanitarian grounds, even if there was no agreement at the United Nations.

In a separate document, the government also laid out its reasons for its conclusion that the Syrian government was responsible for the use of chemical weapons last week, citing an assessment by its Joint Intelligence Committee.

“It is not possible for the opposition to have carried out a CW attack on this scale,” the document said, referring to chemical weapons by their initials. “The regime has used CW on a smaller scale on at least 14 occasions in the past. There is some intelligence to suggest regime culpability in this attack. These factors make it highly likely that the Syrian regime was responsible.”

The document said, however, that “there is no obvious political or military trigger for regime use of CW on an apparently larger scale now, particularly given the current presence in Syria of the U.N. investigation team.”

It added that permission to authorize the use of chemical weapons “has probably been delegated by President Assad to senior regime commanders” but that “any deliberate change in the scale and nature of use would require his authorization.”

It was unclear whether the publication of the two documents would be enough to persuade British lawmakers to endorse the government’s position on Syria, even though the parliamentary motion under discussion does not specifically authorize military action.

The delay in Britain sharpened the focus on France — a European rival for influence — where President François Hollande met on Thursday with Ahmad al-Jarba, the president of the fractured Syrian opposition, which is seeking the overthrow of Mr. Assad. Mr. Hollande’s government was the first in the West to offer formal recognition of the exiled Syrian opposition

In an interview published before the meeting, Mr. Jarba displayed growing impatience with the pace of Western moves toward a military strike and with the level of support for insurgents seeking the overthrow of President Assad.

Asked in the newspaper Le Parisien what he expected of Western military intervention, he replied: “First of all, a punitive strike against the regime. Then political and military support for the Free Syrian Army. For the Assad regime enjoys total support from Russia, Hezbollah and Iran. We lack everything. Our allies have given us nothing of what we want.”

After the meeting, though, Mr. Hollande sounded a cautious tone, saying that “everything must be done for a political solution, but it will only happen if the coalition is able to appear as an alternative with the necessary force, notably from its army.”

“We will only manage this if the international community can put a temporary stop to this escalation in violence, of which the chemical attack is just one example,” Mr. Hollande said.

In Tehran, President Hassan Rouhani was quoted as saying that Iran, Syria’s most powerful regional backer, believed that it was necessary to “apply all efforts to prevent” military action against the authorities in Damascus. “Military action will have a big amount of costs for the region,” Iranian state television quoted Mr. Rouhani as telling President Vladimir V. Putin of Russia in a telephone conversation late Wednesday.

His language seemed outstripped, however, by remarks from Gen. Mohammad Ali Jafari, the commander of Iran’s Revolutionary Guard Corps, who was quoted by the Tasnim news agency as saying an attack on Syria “will mean the imminent destruction of Israel,” Reuters reported. “Syria will become the second Vietnam for the United States,” he said.

While Mr. Rouhani condemned any use of chemical weapons, he also said Iran and Russia would work to prevent any military action against Syria, which he called an “open violation” of international law, The Associated Press reported.

“Early judgment can be dangerous, before clarification” he said, referring to Western assertions that the Syrian authorities had used chemical weapons.

As regional powers maneuvered, there were media reports that warships were being deployed in the Mediterranean by Russia and France, but there was no immediate confirmation of those accounts. The United States Navy has four destroyers within striking range of Syria in the Mediterranean, all of them carrying Tomahawk cruise missiles. Attack submarines also carry Tomahawks and are assumed to be on station in the Mediterranean.

In Moscow, the news agency Interfax quoted an unidentified military source as saying Russia planned to send two warships to the Mediterranean — an anti-submarine vessel and a missile cruiser — because of the “well-known situation.” A French frigate was reported on Thursday to have left the naval base at Toulon on the Mediterranean, but its destination was not immediately clear.

Britain said Thursday that its air force had sent six Typhoon warplanes to a British base at Akrotiri on the Mediterranean island of Cyprus. But, given the government’s promise to delay direct military action, a spokesman for the Ministry of Defense said, the warplanes “are not deploying to take part in any military action against Syria.”

“This is purely a prudent and precautionary measure to ensure the protection of U.K. interests” and the security of the base at Akrotiri, the spokesman said, speaking in return for anonymity under departmental rules. He added that nine Royal Navy vessels were in the Mediterranean as part of an exercise planned before the Syria crisis.

The Syrian government, which has denied accusations by a range of Western and Arab countries that it used chemical weapons in the Aug. 21 attack, moved abruptly on Wednesday to prolong the inspectors’ visit. The authorities announced that they had evidence of three previously unreported chemical weapons assaults that they said had been carried out by insurgents and should be investigated by the inspectors.

If the inspectors look into those accusations, they could remain in Syria well past this weekend, beyond their original mandate, as differences swirling around the conflict there move into ever sharper focus. But Mr. Ban’s reported announcement that the inspectors would leave Syria by Saturday suggested that he wanted them to keep their original schedule.

The American and British governments have said that the existing evidence is persuasive that Mr. Assad’s forces used chemical munitions on civilians in the Damascus suburbs last week, committing what the Obama administration has called a moral atrocity that cannot go unanswered.

The United States could still act without the support of Britain, its closest ally, but the Obama administration has actively sought to build a consensus for a military strike. While expectations had been building that a strike could happen by the weekend, another few days may make no difference to what has been advertised as a short, sharp punishment for the use of chemical weapons, not an effort to oust Mr. Assad!!!