US forces are ready to strike Syria, though the West insists its goal is not regime change but to punish President Bashar al-Assad's government for unleashing chemical warfare on civilians.
The ground for a military intervention was laid out by US Vice President Joe Biden, who for the first time said last week's attack, thought to have killed hundreds, could only have been perpetrated by Assad's forces.
But the likelihood of action within days was met with defiance in Damascus, with regime officials pledging to fight any attack with "surprise" measures, while Syrian allies Russia and Iran warned of dire consequences.
But Britain and France also moved to back the use of force in Syria, while the White House promised to provide declassified evidence this week to prove that last Wednesday's chemical attack was the work of regime forces.
The economic cost also started to be counted, as global stocks dived and world oil prices hit a six-month high.
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Analysts expect to see cruise missiles launched from US and allied submarines, ships and possibly planes, firing into Syria from outside its waters and airspace.
Biden said an "essential international norm" had been violated in Syria.
"There is no doubt who is responsible for this heinous use of chemical weapons in Syria - the Syrian regime," Biden said on Tuesday.
"The president believes and I believe that those who use chemical weapons against defenceless men, women and children should and must be held accountable."
During a news conference earlier on Tuesday, Syria's Foreign Minister Walid Muallem said Damascus will defend itself.
"We have two options: either to surrender, or to defend ourselves with the means at our disposal," he said. "The second choice is the best. We will defend ourselves."
US Defence Secretary Chuck Hagel said the American military is already prepared to act if President Barack Obama gives 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 said in Brunei. "We are ready to go, like that."
French President Francois Hollande says his country is "ready to punish" those behind the chemical attacks and that he would meet the Syrian opposition's leader Thursday.
In London, Prime Minister David Cameron recalled parliament to discuss the crisis and slammed Syria's use of chemical weapons as "morally indefensible".
The White House said that any US action would be to defend the principle that chemical weapons should not be used - and would not aim to topple Assad, despite previous calls for him to go.
"I want to make clear that the options that we are considering are not about regime change," said Obama's spokesman Jay Carney, noting that a declassified version of a US intelligence investigation into the attack would be released this week.
Carney refused to say whether Washington would seek a UN Security Council mandate for action, despite the likelihood of a Russian veto.
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 the chemical weapons stocks themselves, sources in Washington said.
