David Cameron has discussed the Syrian crisis with Russian president Vladimir Putin as Moscow accused Britain and other Western allies of risking aggravating the vicious civil war by launching military action.

The Prime Minister is engaged in a round of diplomatic calls with world leaders amid mounting signs of a likely armed intervention in response to a reported large-scale chemical weapon attack by the regime of president Bashar Assad.

International tensions worsened when UN weapons inspectors came under sniper fire as they sought to investigate last week's deadly attack which killed hundreds in a suburb of Damascus.

Mr Cameron is to chair a meeting of the National Security Council on Wednesday to discuss potential responses to the deadly attack after breaking off his family holiday in Cornwall to return to Downing Street.

Number 10 said a decision would be made on Tuesday over whether to recall Parliament amid increasingly vocal demands from MPs on all sides to be given the chance to pre-approve any UK involvement in military action.

Foreign Secretary William Hague said force may be the only remaining option after the failure of diplomatic efforts to end the bloody civil war in Syria, insisting that it could be deployed legally even without UN Security Council backing. He declined to rule out the possibility of action such as targeted missile strikes being launched within days after Britain, the US, France and Germany agreed the need for firm action to punish the use of chemical weapons.

Russia, a key Assad ally which continues to supply arms to the regime, has consistently vetoed efforts to secure a UN mandate for action, to the frustration of Mr Hague and international allies. Foreign minister Sergei Lavrov said there was no evidence that Assad's forces had used chemical weapons and countries considering military action were acting in breach of an agreement at the G8 conference in June.

Syria's Assad firmly denies "politically motivated" claims that his regime has used chemical weapons - suggesting that opposition forces could be responsible. But Mr Hague said there was "no other plausible explanation" for the deaths and injuries.

Deputy Prime Minister Nick Clegg has cancelled a visit to UK troops serving in Afghanistan so that he can continue to take part in the Government talks over the Syria crisis. A spokesman for Mr Clegg, who is the deputy chair of the National Security Council, said he supported the need for a "strong response" from the international community to the "abhorrent" use of chemical weapons.

Mr Clegg agrees that, while any action would have to be "legal and proportionate", it would not necessarily need UN agreement, he said, as Russia firmly ruled out backing military action and accused the West of taking a "dangerous path".