Syrian opposition leaders were told by Western allies to expect military strikes against Syria's government within days, Reuters news agency reports, as the US, UK and France ramped up rhetoric against President Bashar al-Assad's regime.
According to the report, the Syrian National Coalition was told in clear terms that "action to deter further use of chemical weapons by the Assad government could come as early as in the next few days".
The coalition passed Western allies a list of targets, the report added.
The report on Tuesday came after an alleged chemical attack in Damascus last Wednesday that aid agencies said killed at least 355 people and injured more than 3,000.
The regime denies it carried out the attack.
However, Joe Biden, the US vice president, said that there was no doubt that the Syrian government was responsible for "the heinous use of chemical weapons".
Biden's comments on Tuesday made him the highest-ranking US official to say that the Syrian regime was behind the alleged attack.
No one doubts that innocent men, women and children have been the victims of chemical weapons attacks in Syria. And, there is no doubt who is responsible for this heinous use of chemical weapons in Syria: the Syrian regime.
Biden said the Syrian government was the only actor in the two-year civil war that possesses and can deliver chemical weapons, adding that Assad has blocked UN investigators from the site and has been bombing it for days.
On Tuesday, Chuck Hagel, the US defence secretary, repeated previous statements that US forces were now positioned to strike Assad should the US President Barack Obama give the order.
"We are prepared. We have moved assets in place to be able to fulfill and comply with whatever option the president wishes to take," Hagel said in an interview with the BBC.
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. His government would consider a "proportionate" response that would deter Assad from using chemical weapons in the future, his office said.
France's President Francois Hollande said his country would step up support for Syrian rebels and "punish those who gassed the innocent", while an Arab League statement condemned the chemical attack.
Meanwhile, UN inspectors on Tuesday cancelled a visit to areas affected by the attack over security fears. The team had come under sniper fire as they travelled to one of the sites a day earlier.
White House spokesman Carney said the team's work was "redundant" and that use of chemical weapons had already been established.
Warnings from the east
As Western powers mulled options, Assad's allies Russia and Iran issued fresh warnings against military intervention.
The Russian Foreign Minister Sergey Lavrov said on Tuesday that such an intervention without consulting the United Nations Security Council could have "catastrophic consequences" for the region.
In-depth coverage of escalating violence across Syria
Russia also expressed regret over a decision by the US to postpone talks on Syria scheduled in the Netherlands for Wednesday.
The US said the meeting had been postponed due to "ongoing consultations" over the alleged chemical weapons attack in Syria.
Separately, Iran repeated its opposition to any US attack by saying that a military intervention will engulf the whole region.
"There will definitely be perilous consequences for the region," Abbas Araqchi, an Iranian Foreign Ministry spokesman, said.