British Prime Minister David Cameron's decision to opt out of an attack on Syria left the Obama administration largely on its own as it continued to build its case, at home and internationally, for a coalition-backed military strike to punish the Syrian regime for its alleged use of chemical weapons.

Mr. Cameron argued for British participation, but failed on Thursday to win backing in the House of Commons for even potential military action, falling 13 votes short of a majority after an all-day emotional debate. After the vote, he said he would respect the will of Parliament and not use his prerogative as Prime Minister to authorize British participation in an attack.

Other Western leaders have expressed support for punishing Syria, with Canadian Prime Minister Stephen Harper saying Thursday he is a "reluctant convert" to the need for action. None has offered to take a direct military role. But U.S. officials suggested President Barack Obama is willing to take unilateral action against Syria despite signs of growing resistance from Congress and the lack of a United Nations Security Council mandate.

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"President Obama's decision-making will be guided by what is in the best interests of the United States," White House spokeswoman Caitlin Hayden said in a statement after the British vote. "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."

Mr. Harper, while emphasizing that Canada has no plan to participate in a military strike, said in remarks in Toronto that it would set "an extremely dangerous precedent" if the use of chemical weapons was not met with forceful international action.

"Our government has been a very reluctant convert to the idea that there needs to be some Western military action regarding the Syrian situation," he told reporters. "We have been, and remain concerned when we look at this conflict that it is overwhelmingly sectarian in nature and does not have at present any ideal or obvious outcomes."

The Security Council met briefly on Thursday without taking a vote or issuing any statement. Russia and China have already made it clear they would not support any intervention, and UN Secretary-General Ban Ki-moon called on Western powers to hold off on any military action until chemical-weapons inspectors on the ground in Syria can present their findings.

Speaking in Vienna, Mr. Ban said the UN team is to leave Syria on Saturday morning. Some of the experts will take samples to laboratories in Europe after leaving Damascus, according to UN spokesman Farhan Haq, adding that the team's final report will depend on the lab results and could take "more than days."

Mr. Cameron had been a strong proponent of punishing the Syrian government and told Parliament the gas attack earlier this month that killed hundreds of people in rebel-held areas outside Damascus was "one of the most abhorrent uses of chemical weapons in a century."

He also argued an attack was justified legally even without the backing of the Security Council or 100-per-cent certainty that the government of Syrian President Bashar al-Assad had carried out the chemical attacks. "This is a humanitarian catastrophe," Mr. Cameron said. "It is about the large-scale use of chemical weapons and our response to a war crime, nothing else."

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A legal opinion drafted by the British government said if the Security Council failed to take action, Britain and other countries could use force if there was evidence of "extreme humanitarian distress," no practical alternative and if targeted military action provided "relief of humanitarian need." All three conditions have been met, Mr. Cameron argued.

But opposition had been building in days leading up to the vote and Mr. Cameron had been forced to water down the motion. Even the modified version, which did not specifically commit the country to military involvement, failed.

Germany, too, has expressed opposition to a military strike and Russia has succeeded in blocking efforts by the Security Council to authorize an attack. Syria has denied using chemical weapons and the Assad government has vowed to retaliate if hit.

Overhanging the prospect of military action in Syria is the 2003 invasion of Iraq, which was based largely on what turned out to be false information about Saddam Hussein's possession of weapons of mass destruction. Mr. Cameron acknowledged the "well of public opinion" had been poisoned by the Iraq invasion.

With reports Bill Curry in Ottawa and Associated Press