The US defence secretary says his country is "ready to go" if President Barack Obama orders action in Syria, as the UK recalls Parliament over the crisis.
Britain's Armed Forces are drawing up plans for a potential military intervention in response to an alleged toxic gas attack in Damascus, which is believed to have killed hundreds of civilians.
Rebels in Syria have also handed Western powers a list of suggested targets for a strike, according to a Reuters source.
US defence secretary Chuck Hagel said Mr Obama asked the Pentagon to give him "all options for all contingencies".
Mr Cameron says the Syrian attack was "absolutely abhorrent"
"We are prepared. We have moved assets in place to be able to fulfil and comply with whatever option the president wishes to take," Mr Hagel told the BBC.
US secretary of state John Kerry said there was "undeniable" evidence of a large-scale chemical attack, with intelligence strongly pointing to President Bashar Assad's government.
And he said the regime would be held accountable.
France's President Francois Hollande said his country was prepared to "punish" those who gassed innocent people last Wednesday and it seemed certain that forces loyal to Mr Assad were behind the attack.
Prime Minister David Cameron, who is back in London after cutting short his family holiday, says a clear motion on the crisis will be put before MPs on Thursday.
Mr Hagel says the US is "ready to go"
He wrote on Twitter: "Speaker agrees my request to recall Parliament on Thurs. There'll be a clear Govt motion & vote on UK response to chemical weapons attacks."
Foreign Secretary William Hague has declined to rule out action, such as targeted air strikes, being launched within days.
But Mr Cameron - who is holding a National Security Council (NSC) meeting on Wednesday - is under pressure to be able to legally justify any intervention.
A build-up of military aircraft at Britain's base on Cyprus, RAF Akrotiri, suggested that planning had reached a developed stage.
Sky's Foreign Affairs Editor Tim Marshall said any potential action "would be a punch on the nose for the Assad regime. I don't think the US is ready for Assad to go because they are frightened of who would take his place."
But Syria insists it will not submit quietly to an attack.
Mr Muallem says any foreign strike on his country would be "delusional"
"We have two options: either to surrender, or to defend ourselves with the means at our disposal," said Syrian foreign minister Walid Muallem.
"The second choice is the best: we will defend ourselves."
Mr Muallem also claims Tuesday's UN inspections of alleged chemical attack sites had to be scrapped because of disputes between rebels.
Snipers shot at the UN team on Monday, but the inspectors still managed to collect some "valuable" samples.
One of Britain's bases, RAF Akrotiri, is in southern Cyprus
The UN inspections will take place on Wednesday instead, Mr Muallem said.
Syria denies using the chemical weapons and Russia - which supplies arms to Syria and is the regime's most powerful ally - has backed claims that video footage of victims could be opposition propaganda.
"I challenge those who accuse our forces of using these weapons to come forward with the evidence," Mr Muallem said.
The Arab League has accused the Syrian regime of carrying out the suspected gas attack and Saudi Arabia has called for "firm and serious" action against the state.
The US postponed a Syrian crisis meeting with Russia that was scheduled for this week because of America's ongoing review of the attack.
Snipers shot at UN inspectors on Monday
Russia said postponing The Hague meeting was a "serious disappointment". Moscow also warned that any use of force against Syria would have "catastrophic consequences".
"We call on our American colleagues and all members of the international community to show prudence, strict observance of international law, and above all, the fundamental principles of the UN Charter," the Russian Foreign Ministry said.
And deputy prime minister Dmitry Rogozin claimed western countries are behaving in the Islamic world like a "monkey with a grenade".
Russia's emergencies ministry has told Sky News that it is sending a plane to Syria today to take in humanitarian aid and is planning to bring around 150 of its citizens out.
It is unclear whether Obama would seek authority from the UN or Congress before using force. But it is likely Russia and China would block US efforts to authorise action through the UN Security Council.
Meanwhile, wary investors have exited world markets because of the talk of potential military intervention.
They moved towards safe havens such as gold and bonds while the uncertainty has driven oil prices to a six-month high.