David Cameron has said there is credible evidence of "multiple attacks" using chemical weapons by Syrian President Bashar Assad's regime.
The Prime Minister said the UK believes the "scale of use is sanctioned and ordered by the Assad regime" but also indicated that al Qaida-linked elements in the opposition movement had also attempted to acquire chemical weapons for probable use in Syria.
He restated the Government's position that no decision had been taken to arm moderate rebels opposed to Assad, but added that he was a "brutal dictator who is using chemical weapons under our nose".
Video report by ITV News Washington correspondent Robert Moore:
Earlier Syria's Foreign Ministry said the United States was lying about chemical weapons to give it an "excuse" to intervene in the country's civil war.
And Russia's Foreign Minister Sergei Lavrov told US Secretary of State John Kerry that accusations of chemical weapons use by Assad's forces "are not backed up by verified facts."
Lavrov also said more US military support for Syrian rebels could escalate violence in the region, the foreign ministry added.
Video report by ITV News Middle East correspondent John Ray: