Downing Street has said David Cameron has tonight discussed the situation in Syria in an hour-long video conference with President Obama, French president Francois Hollande, German chancellor Angela Merkel and Italian prime minister Enrico Letta.
Speaking earlier this evening, ITV News political correspondent Carl Dinnen explained the "difficult" conflict between America and Russia over a Syrian intervention - with the former wanting to arm the opposition and the latter already arming the regime:
David Cameron today backed America's claims that the Syrian regime has used chemical weapons.
ITV News Washington correspondent Robert Moore reports on how, after two years of agonizing on the sidelines, America will now provide light weapons to the Syrian rebels - but risks satisfying no one with their level of involvement.
Warning: This video contains images purporting to show victims of chemical attacks which some viewers may find distressing
The White House said it would be dramatically more difficult and costly to set up a no-fly zone over Syria than it was in Libya, stressing that the United States does not have a national interest in pursuing that option.
"We feel like the best course of action is to try to strengthen a moderate opposition," Deputy National Security Advisor Ben Rhodes told a news briefing.
He said the United States did not want to send US troops - or "boots on the ground" - to Syria and said enforcing a no-fly zone over the country could require intense, open-ended US military engagement.
US president Barack Obama will meet with Russian president Vladimir Putin on the sidelines of the G8 meeting in Northern Ireland on Monday and will discuss the crisis in Syria and arms control issues, the White House said today.
"They clearly have a very broad agenda to discuss," said Ben Rhodes, Obama's deputy national security advisor.
Hezbollah leader Sayyed Hassan Nasrallah has vowed that his movement would continue its military support for Syria's President Bashar al-Assad after its fighters spearheaded the recapture of the strategic town of Qusair last week.
"Wherever we need to be, we will be. What we started taking responsibility for, we will continue to be responsible for, and there is no need to give details," he said in a televised speech.
British experts at the Porton Down chemical warfare testing establishment have been involved in the assessment of samples taken from two sites in Syria, the Prime Minister said.
"We believe that the scale of use is sanctioned and ordered by the Assad regime," he said.
"We haven't seen any credible reporting of chemical weapons use by the Syrian opposition."
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-Qaeda-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".
The Prime Minister says elements within al-Qaeda have tried to obtain chemical weapons for use in Syria.
David Cameron says the Syrian regime probably approved use of chemical weapons on 19th March and 13th April.
He says he chose his words carefully on intelligence about al-Qaeda and chemical weapons, given what had happened in the past.
David Cameron said he welcomed the American government's assessment and said it was "consistent" with "credible" evidence he had seen that suggested "multiple" uses of chemical weapons by the Syrian government regime.