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David Cameron said Britain had "made no decision to arm the rebels" in Syria but would continue to provide support in the area.
He described Syria's president Assad as a "brutal dictator."
Syria's Foreign Ministry says the United States is lying about chemical weapons to give it an "excuse" to intervene in the country's civil war.
Foreign Secretary William Hague says the UK agrees "with the US assessment that chemical weapons, including sarin, have been used in Syria by the Assad regime".
Mr Hague added: "The United Kingdom has presented evidence of the use of chemical weapons in Syria to the UN investigation, and we have been working with our allies to get more and better information about the situation on the ground.
"As I said in Washington on Wednesday, the crisis demands a strong, determined and coordinated response from the international community.
"We have to be prepared to do more to save lives, to pressure the Assad regime to negotiate seriously, to prevent the growth of extremism and terrorism, and to stop the regime using chemical weapons against its people."
ITV News spoke to Syrian opposition sources who said military backing from the United States could have the following consequences:
- The government would be unable to re-take Aleppo, bringing an end to military action in the city. Previously government forces successfully took back control of Qusair
- Syrian government, and opposition, would come under pressure to attend a conference in Geneva, aimed at ending the crisis
- The opposition would be able to take control of Deraa, the largest city in southern Syria. That would be a significant victory after the defeat of Qusair
Walid Safur, the British representative of the opposition Syrian National Coalition, says the rebels need anti-aircraft and anti-tank missiles.
The United States is considering setting up a no-fly zone in Syria, possibly close to the border with Jordan, according to Western diplomats.
A senior Russian politician has dismissed claims the Syrian government is using chemical weapons against its own people. Head of the Foreign Policy Committee, Alexei Pushkov, claims the reports have been made up to help the United States justify intervening in the conflict.
Latest ITV News reports
US President Barack Obama has tip-toed over the 'red-line' and armed Syrian rebels but it is a gamble of huge proportions.
Writing for ITV News, international affairs analyst David Butter looks at the timing of US support for Syria and when it may arrive.