U.S. efforts to forge an international coalition to support a military strike on Syria faced fresh uncertainty Thursday, as British Prime Minister David Cameron confronted a political fight in Parliament over military action and France called for a delay until U.N. inspectors finish their work on the ground.

Speaking in the House of Commons, Cameron pledged that Britain would take no action until the United Nations had reviewed a report from weapons inspectors, who are in Syria examining the sites of an alleged chemical attack last week that left hundreds dead.

“It would be unthinkable to proceed if there was overwhelming opposition in the [U.N.] Security Council,” Cameron told Parliament.

At the same time, the prime minister offered an impassioned argument for authorizing military action. He said evidence clearly indicates that the government of Syrian President Bashar al-Assad was responsible for the alleged attack and declared that the West has an obligation to deter a possible repeat of “one of the most abhorrent uses of chemical weapons in a century.”

French officials also are calling for a delay in action until U.N. inspectors conclude their report. “Before acting, we need proof,” said Najat Vallaud-Belkacem, a minister and government spokeswoman, according to Bloomberg News.

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The latest developments suggest that Washington’s allies will insist on waiting at least until next week before launching what initially appeared to be an imminent strike on Syria.

In the meantime, Assad said his country “will defend itself against any aggression,” the state-run Syrian news agency, SANA, reported. As Syrians braced for a possible attack, a top government official told the news service that efforts to stock up on bread and other supplies were “unjustified.”

British officials on Thursday released an intelligence assessment on the alleged chemical attack and a document outlining the legal justification for a robust response.

The three-page intelligence report offered various degrees of certainty about the Assad government’s culpability for the Aug. 21 attack. It said “there are no plausible alternative scenarios to regime responsibility.” But it also categorized the government’s involvement in the alleged attacks as “highly likely,” rather than definite, adding that it is not clear what would have motivated Assad and his deputies to carry out “an attack of this scale at this time.”

Skepticism in Britain and elsewhere is dashing U.S. hopes of quickly securing a broad international coalition.

Cameron was forced to water down the resolution that was debated in Parliament on Thursday, adding a clause that promised a second parliamentary vote in the days ahead. Obama is scheduled to depart next week for Sweden, where he will spend a day before traveling to Russia for a meeting of the Group of 20 major economies.

Labor Party leader Edward Miliband did not rule out supporting military action, but he called for “a calm and measured” analysis, including time for the United Nations to review the inspectors’ report. Recalling the decision a decade ago to invade Iraq based on faulty intelligence about weapons of mass destruction, he told the BBC, “I’m determined we learn the lessons of the past.”

“I do not believe we should be rushed to judgment on this issue based on a political timetable set elsewhere,” Miliband said, in a clear reference to Washington. He added,“I do not rule out supporting the prime minister, but frankly, he needs to make a better case than he has today.”

U.N. Secretary General Ban Ki-moon said the team would finish its work Friday, leave Syria on Saturday and immediately brief him. It was not clear, however, when the team’s findings would be presented to the Security Council.

Russia, a stalwart Assad ally, is a permanent member of the council and is likely to block any resolution endorsing a military strike. But Labor leaders did not specifically ask for backing from the Security Council, only that the council be briefed.

Cameron does not technically need the approval of Parliament to join an international military campaign. But if he does so “without parliamentary approval at this point, it could bring down the government,” said Jason Cowley, editor of the political magazine New Statesman.

In a two-page report released Thursday, the British government said the doctrine of “humanitarian intervention” provides legal justification for military action, even if a resolution endorsing such action were blocked by the U.N. Security Council.

“Military intervention to strike specific targets with the aim of deterring and disrupting further such attacks would be necessary and proportionate and therefore legally justifiable,” the report said.

Karla Adam contributed to this report.