The UK has drafted a resolution and is seeking the backing of the UN Security Council for "all necessary measures to protect civilians" in Syria, said David Cameron
A UK-driven draft resolution will be put to a meeting of the five permanent members of the council in New York on Wednesday evening.
"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," the prime minister said.
Also on Wednesday, a team of UN inspectors in Syria set out on their second day of work examining sites of the suspected chemical weapons attack on 21 August.
A Downing Street spokeswoman said: "Britain has drafted a resolution condemning the attack by the Assad regime, and authorising all necessary measures under Chapter 7 of the UN Charter to protect civilians from chemical weapons. "The resolution will be put forward at a meeting of the five permanent members of the Security Council later today in New York."
The UK's national security council (NSC) is also meeting on Wednesday afternoon to discuss the intelligence gathered by United Nations (UN) inspectors and consider whether or not to push ahead with a military strike against President Bashar Assad.
The meeting comes ahead of Thursday's recall of parliament, when MPs will debate the legality and implications of any possible military strike.
US plans
David Cameron spoke to Barack Obama over the phone several times in the last 24 hours, as both world leaders consider intervention following the death of as many as 1,729 people by poison gas.
The US Vice President Joe Biden said on Wednesday that there is no doubt that Syria's government had used chemical weapons and that it must be held accountable. His comments follow those made by US Secretary of State John Kerry on Tuesday, who said it was "undeniable" that President Assad was behind the attack, which he called a "moral obscenity".
Following the attack last Wednesday, shocking footage emerged of victims of the alleged attack, many of them children.
However the Iranian leader Ayatollah Ali Khamenei warned on Wednesday that American intervention in Syria would be a "disaster for the region." The Iranian state news agency quoted Aytollah Khamenei saying "The region is like a gunpowder store and the future cannot be predicted."
Intervention under scrutiny
Mr Cameron said that any intervention in Syria would not be about the conflict itself, but preventing the use of chemical weapons by any regime.
Decisions about British involvement have not been taken, he said on Tuesday, adding that parliament was the "right place to set out all of the arguments".
He said action must be "proportionate, have to be legal, would have to specifically be about deterring the use of chemical weapons".
The prime minster added that Thursday's debate would ensure "proper" scrutiny and allow the government to listen to MPs.
The NSC includes Defence Secretary Philip Hammond, Home Secretary Theresa May and Deputy Prime Minister Nick Clegg among its members. Foreign Secretary William Hague has warned that Britain cannot allow the use of chemical weapons to go unchallenged, saying Britain "cannot allow diplomatic paralysis to be a shield" for Assad.
Labour leader Ed Miliband has indicated that his party would consider supporting international action, "but only on the basis that it was legal, that it was specifically limited to deterring the future use of chemical weapons, and that any actions contemplated had clear and achievable goals".
The Archbishop of Canterbury has warned MPs not to rush in their decision on whether to vote for military intervention in the Syria conflict. The Most Rev Justin Welby said the impact on those people not directly involved in the fighting was "beyond description and horrible".
Meanwhie a YouGov survey for The Sun revealed that nearly three-quarters of people oppose the deployment of British troops to Syria, and a majority of 3-1 believe the government should be bound by Parliament's vote tomorrow.