David Cameron has clashed with Russian president Vladimir Putin over Syria 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.
He told Mr Putin there was "little doubt" that the government was behind the alleged atrocity and "no evidence" to back the regime's claims that opposition forces were responsible, a Downing Street spokesman said.
"They both reiterated the position agreed by all leaders at the G8 in June: no-one should use chemical weapons and any use would merit a serious response from the international community," he said.
Mr Cameron also said the regime's failure to allow UN inspectors access until five days after the attack suggested they had "something to hide".
But the Russian president told him that there was "no evidence of whether a chemical weapons attack had taken place or who was responsible", Number 10 said.
International tensions worsened when the inspectors came under sniper fire as they sought to investigate last week's deadly attack, which killed hundreds in a suburb of Damascus, on the first day of their agreed visit.
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 tomorrow 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.
"We have no plans to go to war with anyone," he told a press conference, warning the use of force without Security Council backing would be "a crude violation of the international law" like the US-led invasion of Iraq in 2003.
But Mr Hague said the Security Council had failed to "shoulder its responsibilities" on Syria - and could have helped end the conflict "a long time ago" had Russia and China not deployed vetoes.
He said: "Is it possible to respond to chemical weapons without complete unity on the UN Security Council?
"I would argue yes it is - otherwise it might be impossible to respond to such outrages, to such crimes, and I do not think that is an acceptable situation."
Any intervention would be "in accordance with international law and will be based on legal advice to the national security council and to the Cabinet", Mr Hague stressed.
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.
The inspectors were travelling to the site of the attack in a suburb of Damascus when one of their vehicles was "deliberately shot at multiple times by unidentified snipers in the buffer zone area", a UN spokesman said.
They have since replaced the car and made it unharmed into the area where nerve agents are reported to have killed hundreds, where they interviewed survivors, witnesses and doctors and collected samples.
UN secretary-general Ban Ki-moon said he had asked UN disarmament chief Angela Kane to make a "strong complaint" to the regime and opposition forces "so that this will never happen again" and inspectors' security would be assured.
Mr Cameron discussed the crisis with US president Barack Obama for 40 minutes in a telephone call at the weekend, agreeing the need for a "firm response" from the international community.
He also spoke with his French and German counterparts and ordered officials to consider "all options" and his discussion with Mr Putin was among further calls expected to be made today in an effort to secure a co-ordinated response.
Mr Hague told MPs demanding the recall of Parliament from its summer break ahead of any British involvement that it would "depend on the timing and nature of what we propose to do".
"Of course we are conscious of the views of Parliament on these matters and the need to be consulted so we are very conscious of that," he said.
But that failed to dampen cross-party calls for MPs to be brought back before next Monday's scheduled return.
A Downing Street spokesman said: "The Government will decide tomorrow whether the timetable for our response means it will be necessary to recall MPs sooner than Monday when the House is currently due to return."
Shadow foreign secretary Douglas Alexander said: "Both the Foreign Secretary and the Prime Minister have previously made commitments to the House of Commons that Parliament would be recalled before a decision about further UK involvement in Syria was taken.
"This morning's interview by the Foreign Secretary only confirmed the need, before any action is taken, for ministers to make their case to Parliament.
"While of course I understand the Foreign Secretary's reluctance to discuss specific military deployments, he and the Prime Minister do need to be open about the objectives, the legal basis, and the anticipated effect of any possible UK military action in Syria.
"Given both the seriousness of the reported chemical weapons strikes in Syria, and the enduring and complex nature of the conflict itself, ahead of any action being taken I would fully expect the Prime Minister to make his case to Parliament."
Mr Lavrov said military action would go against the terms of the G8 communique issued after June's summit at Lough Erne, Northern Ireland, which said UN inspectors "should make their report and deliver it to the UN Security Council for their assessment".
US secretary of state John Kerry said evidence of the use of chemical weapons was already "real and compelling" but that "additional information" would be made available in the coming days.
In a clear message to Moscow, he said anyone who continued to claim doubts about the footage of the aftermath of the attack "needs to check their conscience and their own moral compass".
He said the shooting of the inspectors' car "further weakens the...credibility" of the regime, which he accused of shelling the area while it was refusing access to destroy evidence.
And though he gave no details of what the response might be, he said Mr Obama "believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people".
"The president will be making an informed decision about how to respond," he told reporters in a statement.
The regime was undoubtedly responsible for a "moral obscenity" that "should shock the conscience of the world", he said.
"By any standard it is inexcusable and despite the excuses and equivocations that some have manufactured, it is undeniable," he said.
"No matter what you believe about Syria, all peoples and all nations who believe in the cause of our common humanity must stand up to ensure that there is accountability for the use of chemical weapons so that it never happens again.
"Anyone who can claim that an attack of this staggering scale could be contrived or fabricated needs to check their conscience and their own moral compass.
In an uncompromising address, he said he had watched the videos of the attack - which quickly circulated on social media but which Russia has said could be opposition propaganda - "one more gut-wrenching time".
The reported number of victims and their symptoms, and first-hand accounts from independent bodies on the ground "all strongly indicate that everything these images are already screaming at us is real - that chemical weapons were used in Syria", he said.
Mr Kerry added: "We have additional information about this attack. That information is being compiled and reviewed together with our partners and we will provide that information in the days ahead.
In his conversation with Mr Putin, Mr Cameron "made clear that there was little doubt that this was an attack carried out by the Syrian regime", Number 10 said.
"There was no evidence to suggest that the opposition had the capability to carry out such a significant attack and the regime had launched a heavy offensive in the area in the days before and after the incident. The regime had also prevented UN access in the immediate aftermath, suggesting they had something to hide," he told him.