US Defense Secretary Chuck Hagel has said the US military is ready to act immediately should President Barack Obama order action against Syria over a chemical weapons attack.

In an interview with the BBC, he said: "We have moved assets in place to be able to fulfill and comply with whatever option the president wishes to take."

Asked if the US military was ready to respond just "like that", Mr Hagel said: "We are ready to go, like that."

White House spokesman Jay Carney said a public version of a formal report by the US intelligence community on the attack will be published this week.

Elsewhere, sources at a meeting between envoys and the Syrian National Coalition said Western powers told the Syrian opposition to expect a strike against President Bashar al-Assad's forces within days.

A source said opposition 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 source, who was at yesterday's meeting in Istanbul, said the opposition was also told that they should still prepare for peace talks at Geneva.

French President Francois Hollande has said there must be a response to the "chemical massacre" in Syria.

He said France is ready to punish those who decided to gas innocent civilians and it will be increasing its military support to the opposition.

British Prime Minister David Cameron has said the country's parliament will be recalled on Thursday to discuss the UK’s response.

Mr Cameron said any intervention in Syria would not be about the conflict itself, but preventing the use of chemical weapons by any regime.

He said: "This is not about getting involved in a Middle Eastern war or changing our stance in Syria or going further into that conflict.

"It's about chemical weapons. Their use is wrong and the world shouldn't stand idly by."

However, he stressed that no decisions about British involvement have been made and said parliament was the "right place to set out all of the arguments".

The White House said tonight that President Obama called Mr Cameron to discuss the situation in Syria.

Earlier, it was announced that Britain's armed forces are drawing up contingency plans for military action.

Mr Cameron's spokesman declined to say whether Britain would wait for United Nations evidence before taking a decision to act.

He will continue talks with international leaders to agree a "proportionate response" that will "deter" President Bashar al-Assad's regime from using toxic agents on the Syrian population, officials said.

A spokesman indicated that a decision could be taken before the results of a report by UN weapons inspectors into the attack is produced, but insisted any response would adhere to international law.

The Syrian government, which denies using gas or obstructing the UN inspectors, said it would press on with its offensive against rebels around the capital.

Foreign Minister Walid al-Moualem said US strikes would help al- Qaeda allies and called Western leaders "delusional" if they hoped to help the rebels reach a balance of power in Syria.

UN chemical weapons inspectors postpone site visit

Elsewhere, the UN inspectors have postponed their next site visit until tomorrow

The UN said: "Following yesterday's attack on the UN convoy, a comprehensive assessment determined that the visit should be postponed by one day in order to improve preparedness and safety for the team.

"Considering the complexities of the site, confirmation of access has not been obtained but is expected later today."

The UN experts came under sniper fire on their way to a site visit yesterday.

UN Secretary General Ban Ki-moon urged all sides in the conflict to give the team safe passage and access.

President Obama has also spoken with Australian Prime Minister Kevin Rudd about possible international responses to the Syrian crisis, while his national security adviser Susan Rice discussed Syria and security with Israeli officials.

Syria attack perpetrators must face justice - Arab League

The Arab League said that those behind the chemical attack in Syria should face international justice.

It called on UN Security Council members to overcome their differences and take action to end the killing there.

In a statement issued after a meeting in Cairo, the 22-member organisation said it held the Syrian government fully responsible for last week's gas attack, in which hundreds were killed.

It also urged the council to "overcome the differences among its members by taking the necessary ... resolutions against the perpetrators of this crime, for which the Syrian regime bears responsibility, and to end the violations and crimes of genocide that the Syrian regime has been carrying out for over two years".