Damascus - The Syrian president's friends and foes on Wednesday headed for a showdown at the United Nations, as Britain pushed for a resolution to pave the way for military strikes over suspected chemical attacks.
Washington and its allies are pressing for likely military action against President Bashar Assad's regime despite stern warnings against intervention from key Damascus supporters Russia and Iran.
Prime Minister David Cameron said London was presenting a resolution "condemning the chemical weapons attack by Assad" to a meeting of the Security Council's five permanent members in New York.
"We've always said we want the UN Security Council to live up to its responsibilities on Syria. Today they have an opportunity to do that," he said via Twitter.
UN Secretary General Ban Ki-moon also urged the Council to "find the unity to act... to use its authority for peace."
But a quick vote on the draft resolution looked unlikely.
Russia, which has already used its veto to block resolutions condemning Syria, said it was premature for the Council to act before a UN team inspecting the sites of the alleged attacks releases its findings.
The UN-Arab League envoy to Syria, Lakhdar Brahimi, confirmed that chemical "substances" were used in the attacks that are thought to have killed hundreds of people on the outskirts of Damascus on 21 August.
Inspectors returned to Damascus on Wednesday afternoon after visiting Eastern Ghouta, a site of one of the reported attacks, and Ban suggested they would need until at least Friday to complete their work.
The UN chief added that their findings would then be analysed and the result sent to the Security Council for "any action they would deem to take".
Brahimi said any military action "must be taken after a decision by the Security Council".
US taking steps
However, such as was the case when Nato forces helped rebels oust Libyan leader Moamer Kadhafi in 2011, there is a precedent for action without a UN resolution.
A senior US official said Washington has ruled out unilateral action and was conferring with allies on potential punitive strikes that could last for more than a day.
"Any military action would not be unilateral. It would include international partners," the official, who spoke on condition of anonymity, told reporters.
US President Barack Obama's deputies are consulting Turkey, Jordan and other partners on contingency plans in preparation for any retaliation by Syria in the event of US-led action, the official said.
"There's a possibility that the Syrian government would use chemical weapons again. I don't think you can discount that," said the official.
US Defence Secretary Chuck Hagel said the American military was already prepared to act if Obama gave the order - though White House aides said no final decision had been taken.
"We have moved assets in place to be able to fulfil and comply with whatever option the president wishes to take," Hagel told the BBC. "We are ready to go, like that."
A military campaign in Syria is expected to be limited in scope, likely to last only several days and to target military sites but not chemical weapons stocks themselves, sources in Washington said.
Russia warned of the consequences of any possible military action.
Foreign Minister Sergei Lavrov was quoted by his ministry as saying "a military solution will lead only to a further destabilisation of the situation in the country and the region."