Britain will on Wednesday submit a draft resolution to the UN Security Council condemning suspected chemical attacks in Syria and demanding "all necessary measures" to protect civilians.

The announcement came as the United States and its allies pressed their case for likely military action against President Bashar al-Assad's regime, despite stern warnings against intervention from Damascus' key allies Russia and Iran.

The UN-Arab League envoy to Syria, Lakhdar Brahimi, confirmed meanwhile that chemical "substances" were used in the attacks that are thought to have killed hundreds of people on the outskirts of Damascus on August 21.

Prime Minister David Cameron said Britain would present a resolution "condemning the chemical weapons attack by Assad" to a meeting of the Security Council's five permanent members in New York on Wednesday.

"We've always said we want the UN Security Council to live up to its responsibilities on Syria. Today they have an opportunity to do that," Cameron said via Twitter.

UN Secretary General Ban Ki-moon also urged the Council to unite for peace.

"The Council must at last find the unity to act. It must use its authority for peace," Ban said.

The developments came after US Vice President Joe Biden said the chemical attacks could only have been perpetrated by Assad's forces.

"There is no doubt who is responsible for this heinous use of chemical weapons in Syria -- the Syrian regime," said Biden.

"The president believes and I believe that those who use chemical weapons against defenceless men, women and children should and must be held accountable."

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.

But despite US reports possible strikes could begin as early as Thursday, a French said the country's parliament would only debate the Syria crisis on September 4, raising doubts about any imminent military action.

Syria's Foreign Minister Walid Muallem has already declared that Damascus would be able to defend itself.

"We have two options: either to surrender, or to defend ourselves with the means at our disposal," he said on Tuesday. "The second choice is the best. We will defend ourselves."

Syria's ambassador to the United Nations also hit back at accusations of responsibility for chemical weapons strikes.

"Many facts tend to prove the innocence of the Syrian government, which has been subject to false accusations," ambassador Bashar al-Jaafari told state media.

Jaafari said such facts also showed that "armed groups have used chemical weapons in order to bring about military intervention and aggression against Syria".

US Defence 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 have moved assets in place to be able to fulfil and comply with whatever option the president wishes to take," Hagel told the BBC. "We are ready to go, like that."

French President Francois Hollande said his country was "ready to punish" those behind the chemical attacks and that he would meet the leader of Syria's main opposition bloc on Thursday.

But Russia, which has already used its Security Council veto to block resolutions condemning the Syrian regime's conduct in the country's 29-month conflict, warned of the consequences of any possible military action.

Foreign Minister Sergei Lavrov said "a military solution will lead only to a further destabilisation of the situation in the country and the region," the Russian foreign ministry said.

And supreme leader Ayatollah Ali Khamenei of Iran, the Assad regime's chief regional ally, warned on Wednesday that "US intervention will be a disaster for the region," state television reported him as saying.

"The region is like a gunpowder depot. (Its) future cannot be predicted" in case of a military strike against Syria, he added.

UN inspectors based in Syria, meanwhile, left their Damascus hotel on Wednesday to head to Eastern Ghouta, a site of one of the alleged chemical attacks outside the capital.

This came a day after the experts suspended their mission for one day over safety concerns.

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.

An official in Syria's main opposition National Coalition said the group expects a Western military intervention and it has been consulted over targets, which included airports, military bases and arms depots.

"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."

Syria's conflict erupted in March 2011 with peaceful anti-regime protests but, following a brutal crackdown on those demonstrations, it evolved into a war that has killed more than a 100,000 people.