The US defence secretary says his country is "ready to go" if Barack Obama orders action in Syria, as the UK recalls its parliament over the crisis.
British Armed Forces are drawing up plans for a potential military intervention in response to an alleged toxic gas attack in Damascus, which is said to have killed 1,300 people.
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".
"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.
The British Prime Minister, 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.
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."
The UK 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 British National Security Council meeting tomorrow - 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.
UN Secretary-General: "If proven, any use of chemical weapons by anyone under any circumstances is a serious violation of international law and an outrageous crime"
But Syria insists it will not submit quietly to an attack.
"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 today's UN inspections 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. The UN inspections will take place on Wednesday instead, Mr. Muallem said.
Earlier, US Secretary of State John Kerry said Syria's use of chemical weapons was "undeniable". Syrian President Bashar Assad's would be held accountable, Mr. Kerry said.
The US also postponed a Syrian crisis meeting with Russia that was scheduled for this week because of America's ongoing review of the attack.
Russia - the Syrian regime's most powerful ally - 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.
Syria denies using the chemical weapons and Moscow - which supplies arms to Syria - 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 today.
