UN weapons inspectors have been ordered to leave Syria early amid mounting anticipation of US-led military strikes.

As the five permanent members of the security council held a second emergency meeting on Syria in two days on Thursday evening, the UN secretary general, Ban Ki-moon, instructed the 20-strong inspection team in Damascus to leave on Saturday, a day ahead of schedule. Ban also announced that the team would report to him immediately on departure, raising the possibility that the UN could issue an interim report on the 21 August chemical attacks that left hundreds of people dead.

The inspectors had not been due to deliver their findings for a week at least, with the analysis of samples a painstaking task. The demand for a rushed early assessment reflects the fraught atmosphere at the UN triggered by US threats to launch punitive air strikes within days.

In Washington US intelligence officials were on Thursday seeking to persuade congressman of the evidence that the Syrian government was responsible for chemical weapons attacks, as the Obama administration resisted comparisons with the run-up to the Iraq war.

Congressmen, currently in recess, are due to participate in "unclassified briefing" via telephone conference.

Downing Street had suggested it received US assurances that the White House was willing to wait until Tuesday to give the House of Commons time for a second debate on British involvement in air strikes, but Whitehall sources have acknowledged it is possible the US could go ahead with an attack on Syria without Britain, possibly with France, where the president, François Hollande – another proponent of punitive action – is not constrained by a parliamentary system.

Josh Earnest, the White House's deputy spokesman, asked about the possibility of "going it alone" at the daily briefing, repeatedly said it was in US "core national security interests" to enforce international chemical weapons norms. Earnest also provided the first formal confirmation of the kind of military offensive being considered. "What we're taking about here is something that is discrete and limited."

US officials told their British counterparts they understood the demands of the UK parliamentary system, and set great store in the transatlantic "special relationship" but that other considerations including core American national interests could take precedence when Barack Obama made a final decision how to act.

It is understood that one of the factors involved in the discussions within the Obama administration was the need to uphold US credibility, particularly in anticipation of a looming confrontation with Iran over its nuclear ambitions.

The president appeared to hint at such considerations in a PBS television interview on Wednesday. "If we are saying in a clear and decisive but very limited way, we send a shot across the bow saying: 'Stop doing this,' this can have a positive impact on our national security over the long term," he said.

Obama said he not made a final decision on air strikes but added: "We have concluded that the Syrian government in fact carried these out."

Most observers said that US air strikes were still likely, even without UK participation, probably in the form of cruise missiles launched from American ships in the Mediterranean. But the parliamentary revolt in the UK and tepid support for military action in the US, provoked the first real doubts for several days that the Obama administration would carry out its threats.

"If the administration can't even count on the full-throated support of our closest ally, the country that stuck by us even during the worst days of Iraq, that legitimacy is going to be called into question," said Ken Pollack, a Middle East expert at the Brookings Institution thinktank.

Meanwhile, US and British government attempts to rally support for military action were damaged by the publication of an Associated Press report quoting unnamed US intelligence officials as describing the case against the Assad regime as no "slam-dunk", adding that uncertainty remained over real control over Syrian chemical weapons stockpiles.

The basketball term slam-dunk, reflecting certainty of outcome, has haunting overtones in Washington as it was attributed to the then CIA director George Tenet in describing the intelligence case in the runup to the 2003 Iraq invasion for Saddam Hussein's supposed possession of weapons of mass destruction. The AP report conflicts with the assertion by the UK joint intelligence committee on Thursday that "it is not possible for the opposition to have carried out a CW [chemical weapons] attack on this scale".

The report quoted "multiple US officials" as pointing towards gaps in the US intelligence picture, in strong contrast to the certainty expressed by American leaders, including Barack Obama, and said the US intelligence evidence against Syria was "thick with caveats". "It builds a case that Assad's forces are most likely responsible while outlining gaps in the US intelligence picture," AP reported.

Thursday night's second meeting of the permanent five security council members – the US, UK, France, Russia and China – was called by Russia, which is adamantly opposed to military intervention. An initial discussion of a British resolution calling for a UN mandate for armed action foundered on Wednesday in the face of Russian opposition.

In was not clear whether the Russians intended to bring new proposals to the table. The call for a meeting followed a telephone conversation between Angela Merkel and Vladimir Putin, in which the German chancellor called on the Russian leader to seek a consensus alternative at the UN to military action.

"The chancellor called on the Russian president to use negotiations in the UN security council for a quick, unanimous international reaction," a German government statement said after the call.

The accelerated departure of the UN weapons inspectors was reminiscent of similar hasty exit from Iraq more than a decade ago, after receiving a tip-off from western intelligence agencies that US air strikes against Saddam Hussein's regime were imminent.

The inspectors, who have spent three days in the eastern Damascus suburbs where the 21 August mass killing of civilians took place, had so far sent none of the samples they had collected out of the country for tests, but would be carrying the samples with them when they left. Laboratory work would only begin once they had left Syria.

It is possible however Ban will ask for an interim report that could provide circumstantial evidence such as witness testimonies and the identification of the weapons and delivery systems used in the attack. That could provide part of what the UN has described as an "evidence-based narrative", from which member states could draw their own inferences. It is not clear whether the head of the inspection mission, Åke Sellström, would agree to such a partial report under intense pressure from the secretary general and some security council members.