US Vice President Joe Biden said there is no doubt that Syria's government was responsible for a recent chemical attack in Damascus, as Western allies ramp up rhetoric in preparation for possible military strikes against Syria.
Biden's comments on Tuesday made him the highest-ranking US official to say the Syrian government was behind the alleged chemical weapons attack on August 21, that aid agencies said killed at least 355 people and injured more than 3,000.
"[...] There is no doubt that an essential, international norm has been violated. Chemical weapons have been used. Everyone acknowledges their use. No one doubts that innocent men, women and children have been the victims of chemical weapons attacks in Syria.
"And there's no doubt who is responsible for this heinous use of chemical weapons in Syria: the Syrian regime," Biden said at an American Legion's national convention.
Biden said the Syrian government was the only actor in the two-year civil war that possessed and could deliver chemical weapons, adding that President Bashar al-Assad has blocked UN investigators from the site and has been bombing it for days.
"The president believes and I believe that those who use chemical weapons against defenceless men, women, and children should and must be held accountable," Biden said.
US-based Foreign Policy magazine reported on Tuesday that US intelligence services overheard a Syrian defence ministry official in "panicked phone calls with the leader of a chemical weapons unit" after last week's deadly chemical attack.
"Last Wednesday, in the hours after a horrific chemical attack east of Damascus, an official at the Syrian Ministry of Defense exchanged panicked phone calls with the leader of a chemical weapons unit, demanding answers for a nerve agent strike that killed more than 1,000 people," Foreign Policy reported.
"Those conversations were overheard by US intelligence services," the magazine said in a statement. "That is the major reason why American officials now say they're certain that the attacks were the work of the Bashar al-Assad regime - and why the US military is likely to attack that regime in a matter of days."
The report came as the Reuters news agency reported that the Syrian National Coalition, a rebel political alliance, was told in clear terms that "action to deter further use of chemical weapons by the Assad regime could come as early as in the next few days".
The coalition passed Western allies a list of targets, the report added.
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 the August 21 chemical attack was the work of regime forces.
The White House said President Barack Obama has not settled on how to respond to the attack. The Pentagon said US military forces are ready to strike Syria if Obama gives the order.
'Ready to strike'
On Tuesday, Chuck Hagel, the US defence secretary, repeated previous statements that US forces were now positioned to strike Assad should the decision to take military action be made.
The US has stationed warships armed with cruise missiles in the Mediterranean, and has air bases across the Middle East.
David Cameron, the British prime minister, announced the recall of parliament from summer recess for a debate on Syria on Thursday.
In-depth coverage of escalating violence across Syria
His government would consider a "proportionate" response that would deter Assad from using chemical weapons in the future, his office said.
France, meanwhile, has said that it is "ready to punish" those who carried out the attack, according to a statement made by President Francois Hollande on Tuesday.
Meanwhile, Australia, the incoming chair of the UN Security Council, endorsed possible military action against Syria over the use of chemical weapons, even if the council fails to agree on action.
Australia, a close ally of the US, is due to take over the rotating leadership of the council on Sunday, a role that requires it to assist council members to reach agreement.
But Foreign Minister Bob Carr said that if it was proved the Syrian regime had used chemical weapons, the world had a mandate to respond, even if the United Nations failed to agree on such action.
"We're moving to a stage where America and like-minded countries are contemplating what sort of response," Carr told reporters on Wednesday.
The onrushing likelihood of military action within days was met with defiance in Damascus, with government officials pledging to fight any attack with "surprise" measures, while Syrian allies Russia and Iran warned of dire consequences.
UN Secretary General Ban Ki-moon, meanwhile, has said through a spokesperson that his organisation's focus remains on "a diplomatic solution".