US forces readied to strike Syria on Tuesday, as the West insisted its goal was not regime change but to punish the Assad government for unleashing chemical warfare on civilians.
The White House said there was "no doubt" that President Bashar al-Assad's forces were to blame for an attack which killed hundreds of Syrians last week, and promised to provide declassified evidence this week to prove it.
With military action seen as a near certainty and expected within days, Syria vehemently vowed to defend itself with what it dubbed "surprise" measures, while allies Russia and Iran warned that the use of force would have dire consequences.
Global stock markets dived and world oil prices hit a six-month high as the drumbeat of war grew louder.
US Defense Secretary Chuck Hagel said the American military was already prepared to act if President Barack Obama gave the order -- though White House aides said no final decision had been taken.
"We are prepared. 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."
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.
US allies like Britain and France could also be involved.
US Vice President Joe 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 defenseless men, women and children should and must be held accountable."
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.
Carney said that claims that it was unclear who fired the weapons or that Syrian rebels could be responsible were "preposterous."
It was undeniable that chemical weapons were used, that the Syrian regime has custody of the country's chemical arsenal and that it used the type of rockets bearing the murderous payload in last Wednesday's attack, he said.
Carney added that a declassified version of a US intelligence investigation into the attack would be released this week.
He refused to say whether Washington would seek a UN Security Council mandate for action, despite the likelihood of a Russian veto.
A Syrian campaign 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.
An opposition Syrian National Coalition official said in Beirut that the group expects a Western military intervention within days and has been consulted over targets.
"It's a question of days and not weeks," said Ahmad Ramadan, adding that "there have been meetings between the Coalition, the (rebel) Free Syrian Army and allied countries during which possible targets have been discussed."
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They included airports, military bases and arms depots, he said.
During a defiant news conference earlier on Tuesday, Syria Foreign Minister Walid Muallem said Damascus would 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 allies kept up a coordinated push for action.
President Francois Hollande said France was "ready to punish" those behind the attacks and will meet the Coalition's leader Thursday.
In London, Prime Minister David Cameron recalled parliament to discuss the crisis and said Syria's use of chemical weapons was "morally indefensible."
His deputy Nick Clegg echoed US assurances about not seeking regime change.
The Arab League meanwhile put the "entire responsibility" for the "horrible crime" in Syria on Assad's government.
The regime has denied it fired chemical weapons into the Damascus suburb, killing many of the victims in their beds. It says the rebels battling Assad in a vicious civil war are responsible.
Syria's ally Moscow, meanwhile, stepped up its anti-Western rhetoric.
"Attempts to bypass the Security Council, once again to create artificial groundless excuses for a military intervention in the region are fraught with new suffering in Syria and catastrophic consequences for other countries of the Middle East and North Africa," a Russian foreign ministry spokesman said.
Deputy Prime Minister Dmitry Rogozin, said more concisely on Twitter that: "The West behaves towards the Islamic world like a monkey with a grenade."
Iran, Syria's main regional ally, said Western action would threaten the stability and security of the region.
But another key regional power Turkey, said the chemical attack was a "crime against humanity" that "cannot go unpunished."
Amid fears of reprisals for any Western attack, Israeli Prime Minister Benjamin Netanyahu vowed a fierce retaliation if his country came under fire.
"We are not part of the civil war in Syria, but if we detect any attempt to hurt us, we will react, and react fiercely," he said.
Increasing signs of impending military action hit global markets.
Most European equities fell sharply as investors ignored solid data from Germany and nervously eyed Syria, sending the price of safe-haven gold soaring.
London's FTSE 100 index slid 0.79 percent to 6.440.97 points.
The Dow Jones Industrial Average closed down 170.56 points (1.14 percent) at 14,775.90.
Oil prices soared, with Brent crude striking six-month highs on supply concerns.