Britain, the US and France helped "terrorists" use chemical weapons in Damascus, Syria's deputy foreign minister has claimed.
"The terrorist groups are the ones who used them with American, British and French encouragement. This encouragement should stop," said Faisal Al-Miqdad.
Foreign Secretary William Hague said "all the evidence" pointed to Bashar al Assad's regime being behind the chemical weapon attack, a response to which he said would be discussed at a meeting of the five permanent members of the United Nations Security Council.
"We believe that it's time the United Nations Security Council shouldered its responsibilities on Syria, which for the last two and a half years it has failed to do," he said.
"We're clear that if there isn't agreement at the United Nations, we and other nations still have a responsibility on chemical weapons.
"We have to confront something that is a crime against humanity. If we don't do so we will have to confront even bigger war crimes in the future.
David Cameron cut short his holiday to return for the talksHe repeated David Cameron's statement that the National Security Council (NSC) had "agreed unanimously that the use of chemical weapons by Assad was unacceptable - and the world should not stand by".
Military commanders in the council, which held talks chaired by the Prime Minister at Downing Street, were helping draw up plans for missile strikes against Syria.
The NSC also "agreed unanimously on a recommendation" to be considered by the Cabinet tomorrow, while the UN Security Council meeting in New York later will look at Britain's draft resolution condemning the attack and "authorising all necessary measures".
UN Secretary-General Ban Ki-moon has urged all sides to prioritise a diplomatic solution and said his team needs until Sunday to establish the full facts of the alleged chemical attack.
This afternoon's NSC meeting at Downing Street was expected to discuss the intelligence gathered by UN inspectors from their initial visit to Mouadamiya, the site of last week's suspected chemical weapons attack that allegedly killed more than 1,300.
General Sir Nick Houghton, chief of the defence staff, was also expected to outline a series of options for targeted attacks.
A UN weapons inspector visits wounded Syrians after the alleged gas attackIt is understood the most likely military response would be a strike launched from US Navy warships against targets such as command and control bunkers.
The US Navy is repositioning several vessels in the eastern Mediterranean, including four cruise missile-carrying destroyers and a missile-firing submarine.
Military analysts have also suggested a British Trafalgar class submarine might be used as a potential launch platform.
However, some experts remain sceptical about whether limited strikes on tactical targets would provide a strong enough deterrent.
Defence analyst Francis Tusa told Sky News: "I'm not necessarily sure it puts any particular pressure on the regime to change its behaviour.
"Losing the odd bit of hardware that the Russians will replace for free doesn't seem to be that much of value."
Iran's Supreme Leader, Ayatollah Ali Khamenei, has also responded to the rising tensions, reportedly saying that US military intervention would be "a disaster for the region".
"The region is like a gunpowder store and the future cannot be predicted," Iran's ISNA agency quoted him as saying.
More than 1,300 are said to have died as the result of the alleged attackTurkey and Iraq both say they have placed their military on high alert.
Mr Cameron and US President Barack Obama have agreed that "all the information available confirmed a chemical weapons attack had taken place", a Downing Street spokesperson said on Wednesday morning.
"They both agreed they were in no doubt that the Assad regime was responsible," said Number 10.
Nato has also given its support for tough action against Syria, "condemning in the strongest possible terms these outrageous attacks".
"Those responsible must be held accountable," it added.
Parliament will be recalled on Thursday for a final vote on what action the UK should take.
Sky sources say a government motion is expected to call for "appropriate measures" but will not contain a timetable for action.
Foreign Secretary William Hague was at the NSC meetingSpeaking yesterday, Mr Cameron said action must be "proportionate, have to be legal, would have to specifically be about deterring the use of chemical weapons".
Decisions about British involvement have not been taken, he said on Tuesday, adding Parliament was the "right place to set out all of the arguments".
"We shouldn't stand by when we see this massive use of chemical weapons and appalling levels of suffering," he said.
"But I would say this to people - there is never 100% certainty, there is never one piece or several pieces of intelligence that give you absolute certainty.
"But what we know is this regime has huge stocks of chemical weapons. We know they have used them on at least 10 occasions prior to this last widescale use."
Labour leader Ed Miliband has indicated that his party would consider supporting international action if it was legal and had "clear and achievable goals".
While political momentum towards intervention mounts, the British public has yet to be persuaded.
A child is treated after the alleged chemcial attackA YouGov survey for The Sun revealed that nearly three-quarters of people oppose the deployment of British troops to Syria.
And a majority of 3-1 believe the Government should be bound by Parliament's vote tomorrow.
UN weapons inspectors arrived at the site of the alleged attacks on Wednesday morning, a day after suspending their mission over safety concerns.
The inspectors came under sniper fire when they began their operation on Monday.
Russia has confirmed it has started to pull its citizens out of Syria as the likelihood of military action increases.
It flew 89 people out of the country on Tuesday night and 28 more on Wednesday morning.
Russia and China both strongly oppose the intervention, with Russian Foreign Minister Sergei Lavrov believing it would seriously destabilise the region.