Barack Obama's plans for air strikes against Syria were thrown into disarray on Thursday night after the British parliament unexpectedly rejected a motion designed to pave the way to authorising the UK's participation in military action.

The White House was forced to consider the unpalatable option of taking unilateral action against the regime of Bashar al-Assad after the British prime minister, David Cameron, said UK would not now take part in any military action in response to a chemical attack in the suburbs of Damascus last week.

Although Britain's support was not a prerequisite for US action, the Obama administration was left exposed without the backing of its most loyal ally, which has taken part in every major US military offensive in recent years.

Caitlin Hayden, a spokeswoman for Obama's national security council, indicated the administration would consider acting unliaterally. "The US will continue to consult with the UK government – one of our closest allies and friends. As we've said, President Obama's decision-making will be guided by what is in the best interests of the United States.

"He believes that there are core interests at stake for the United States and that countries who violate international norms regarding chemical weapons need to be held accountable."

The US appears to have taken British support for granted. Hours before the vote, the chairman of the Senate intelligence committee, Diane Feinstein, expressed confidence that Britain would join any strike.

Feinstein, a Democrat and staunch administration ally, told Time magazine: "I think the UK makes a difference. I think if the president were to decide to go there's a very high likelihood that the United Kingdom would be with us."

The timing of the British vote, 272 to 285 against the government, was disastrous for Obama. Less than 30 minutes after the vote, senior intelligence officials began a conference call with key members of Congress, in an attempt to keep US lawmakers on side.

Congressional leaders and the chairs and ranking members of national security committees were briefed by the most senior US intelligence officials, amid signs that some of the support for military strikes against Syria was fading.

The officials said there was "no doubt" that chemical weapons were used in Syria last week, Reuters reported. Obama aides cited intercepted communications of Syrian officials and evidence of movements by Syria's military around Damascus before the attack that killed more than 300 people, said Eliot Engel, the top Democrat on the House foreign affairs committee.

The 90-minute briefing was conducted by secretary of state John Kerry, secretary of defense Chuck Hagel, national security adviser Susan Rice, among others.

After the briefing, Carl Levin, the Democratic chairman of the Senate armed services committee, urged a cautious approach. "I have previously called for the United States to work with our friends and allies to increase the military pressure on the Assad regime by providing lethal aid to vetted elements of the Syrian opposition.

"Tonight, I suggested that we should do so while UN inspectors complete their work and while we seek international support for limited, targeted strikes in response to the Assad regime's large-scale use of chemical weapons against the Syrian people."

The UN has said more time should be given to diplomacy, and France, which earlier this week declared its support for taking action against Syria, is now calling for more time so UN inspections can be completed. A session of the United Nations security council in New York, called by Russia, broke up without agreement.

Ban Ki-moon, the UN secretary general, 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. 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.

Shortly before Britain's parliamentary vote, the New York Times quoted senior administration officials saying the US administration was prepared to launch strikes on Syria without a UN security council mandate or the support of allies such as Britain.

Earlier on Thursday, Joshua Earnest, the White House deputy spokesman, seemed to confirm that was a possibility when he was asked whether the US would "go it alone". He repeatedly said it was in US "core national security interests" to enforce international chemical weapons norms. "The president of the United States is elected with the duty to protect the national security interests of America," he said. Any strikes would be "discreet and limited", he said.

However, Earnest also stressed the broad international support for the US position – backing that now appears to be dissipating. The Arab League has blamed Syria for the chemical attack, but stopped short of advocating punitive strikes by the US.

In recent days, Obama has spoken personally with leaders of France, Australia, Canada and Germany. But none were as important as Britain, a traditional ally during US military actions which has been lobbying behind the scenes for months for a tougher action on Syria.

Ken Pollack, a fellow from the Saban Centre for Middle East Policy, said that with continuing uncertainty over the intelligence picture, and no obvious legal mandate for military action, the US will be desperate to secure more international backing to argue that intervention is "legitimate".

"If the administration can't even count of 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," he said.

Now that the UK parliament has rejected an attack on Syria, Washington's space for planning one is likely to be constrained, particularly as the Obama administration prepares to release its intelligence tying Assad to the 21 August gas attack. An unclassified report is due to be published on Friday.

Paul Pillar, a former senior CIA Middle East analyst and Georgetown professor, said the loss of British support would lead to more "intense" scrutiny of the US case for action against Syria. "The UK is, in many important respects, the most important ally of the United States," said Pillar. "This action by parliament is unquestionably significant in that regard."