Bashar al-Assad's friends and foes headed for a showdown at the United Nations Wednesday, as Britain pushes for a resolution to pave the way for military strikes over suspected chemical attacks.
The meeting comes as the United States and its allies pressed their case for likely military action against the Syrian president's regime, despite stern warnings against intervention from key Damascus supporters Russia and Iran.
Prime Minister David Cameron said Britain would present a resolution "condemning the chemical weapons attack by Assad" to a meeting of the Security Council's five permanent members in New York on Wednesday.
"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 the prospects for a quick, much less positive, vote on the draft resolution look dim.
Close Damascus ally Russia, which has already used its veto to block resolutions condemning Syria, said Wednesday it was premature for the Council to act before a UN team inspecting the sites of the alleged attacks releases its findings.
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 August 21.
And inspectors headed Wednesday to Eastern Ghouta, a site of one of the reported attacks, after delaying their work for a day over security concerns.
Brahimi added that any military action must have UN approval.
"I think international law is clear on this. International law says that military action must be taken after a decision by the Security Council," he said.
However, such as is the case when NATO forces helped rebels oust Libyan leader Moamer Kadhafi in 2011, there is a precedent for acting without the United Nations.
A senior US official said Washington has ruled out unilateral action and is 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 senior administration official, who spoke on condition of anonymity, told reporters.
US President Barack Obama's deputies are holding discussions with 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.
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But if the United States took no military action, then it would send a dangerous signal to other regimes with chemical stockpiles, including North Korea, the official said.
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.
French President Francois Hollande said his country was "ready to punish" those behind the chemical attacks and that he would meet the leader of Syria's main opposition bloc on Thursday.
But Russia warned of the consequences of any possible military action.
Foreign Minister Sergei Lavrov was quoted by the ministry as saying "a military solution will lead only to a further destabilisation of the situation in the country and the region".
And supreme leader Ayatollah Ali Khamenei of Iran, the Assad regime's chief regional ally, warned on Wednesday that "US intervention will be a disaster for the region," state television reported him as saying.
"The region is like a gunpowder depot. (Its) future cannot be predicted" in case of a strike on Syria, he added.
The developments came after US Vice President Joe Biden said the chemical attacks could only have been perpetrated by Assad's forces.
"There is no doubt who is responsible for this heinous use of chemical weapons in Syria -- the Syrian regime," said Biden.
Analysts expect to see cruise missiles launched from US and allied submarines, ships and possibly warplanes, firing into Syria from outside its waters and airspace.
Syria's UN ambassador hit back at the accusations against the regime, repeating an assertion that the alleged attacks had been carried out by rebels.
"Many facts tend to prove the innocence of the Syrian government, which has been subject to false accusations," Bashar al-Jaafari told state media.
Such facts also showed that "armed groups have used chemical weapons in order to bring about military intervention and aggression against Syria," he said.
His remarks came as rebel fighters said they fired Katyusha rockets at government positions in central Damascus on Wednesday in retaliation for the alleged chemical attacks on civilians.
The conflict erupted in March 2011 with peaceful anti-regime protests but, following a brutal crackdown on those demonstrations, it evolved into a war that has killed more than 100,000 people and created millions of refugees.