David Cameron and President Barack Obama spent 40 minutes discussing the crisis, ordering officials to examine “all options”.
A decision is due within 48 hours while military strategists draw up a list of potential targets which would cause most damage to President Bashar Assad’s chemical warfare capability.
Meanwhile, Royal Navy vessels were being prepared for possible action as Foreign Secretary William Hague called for a firm response.
“We cannot in the 21st century allow the idea that chemical weapons can be used with impunity, that people can be killed in this way, and there are no consequences,” he warned.
“So we believe it is very important that there is a strong response.”
Weapons experts will visit the site of the alleged deadly poison gas attack today amid stark warnings from Syria that military intervention would turn the Middle East into a “ball of fire”.
A team of United Nations inspectors is expected to move in after Assad’s regime pledged to maintain a ceasefire during the fact-finding mission to the site in a Damascus suburb.
However, Washington accused the Syrian government of having “something to hide” and delaying access for four days to cover up evidence.
Aid agencies put the casualty figure from the attack at more than 350 people killed.
As well as the warning from the Assad regime about the effects of a military strike, Iran told the US not to cross the “red line” on Syria, saying it would have “severe consequences”.
Britain spearheaded an intense round of diplomacy as Mr Cameron held telephone conversations with both President Obama and Francois Hollande of France. The Prime Minister and President Hollande are insisting that the “crime must not be swept under the carpet”.
Russia, a key ally of Syria, welcomed the decision to allow access to the UN inspectors but warned the West against pre-empting the results.
We cannot in the 21st century allow the idea that chemical weapons can be used with impunity
But Mr Cameron faces domestic resistance to military action from backbenchers.
Tory MPs have already insisted there should be a Commons vote before arms are supplied to rebels. Conservative MP Bob Stewart said that Parliament must be recalled first.
The US has so far been reluctant to use any force against the Damascus regime.
Defence Secretary Chuck Hagel said the military was ready to exercise “options” on Syria should force be called for, but declined to say what those might be.
Hundreds of people were poisoned on Wednesday. Medecins Sans Frontieres said hospitals in Syria treated some 3,600 patients with “neurotoxic symptoms’, 355 of whom died. Medical staff described people suffering from convulsions, extreme salivation, contracted pupils and sight and respiratory problems.
However, the French organisation stressed it could not “scientifically confirm” the use of chemical weapons.
Labour Shadow Foreign Secretary Douglas Alexander said: “It is right the UK works with the international community to find a way forward.
“But if Mr Cameron is considering military options involving UK personnel then I’d expect him to seek a recall of Parliament and make his case in advance of a decision being made.”
Downing Street said summoning the Commons from its summer break had not been ruled out – but stressed Mr Cameron “reserved the ability to take action very swiftly if needed”.