The people of Syria risk "Armageddon" if the international community fails to respond robustly to the use of chemical weapons by the regime of President Bashar Assad, David Cameron suggested today.
The Prime Minister told the House of Commons it would be "perilous" for the world to step back from Barack Obama's warning that chemical weapons use was a "red line" issue, as it might embolden Assad to repeat the deadly attacks.
Downing Street denied that Cameron was seeking to send a message to the US president to press ahead with military strikes against the Assad regime, insisting that this was a decision for Washington to make.
His warning came as Obama said the credibility of the international community was "on the line" unless it enforces the provisions of the Chemical Weapons Convention, which has been signed by 98% of the world's nations.
Speaking in Sweden a day before the G20 summit of world leaders in St Petersburg, hosted by Russian president and close Assad ally Vladimir Putin, Mr Obama said: "I didn't set the red line - the world set the red line...
"My credibility isn't on the line. The international community's credibility is on the line, and America and Congress's credibility is on the line, if we give lip service to the idea that these norms are important."
On the eve of the St Petersburg summit, Putin warned that Western military action without United Nations backing would be an act of aggression.
But he said he "doesn't exclude" Moscow voting in favour of a military response at the Security Council if evidence is produced proving the regime was behind the attack.
It was "absolutely absurd" to assume Assad's forces would risk the international consequences of using banned weapons when they were on the front foot in the battle with opposition forces for that part of Damascus, he said.
There needed to be "evidence that would be obvious and prove beyond doubt who did it and what means were used".
Obama said he remained convinced that the regime was to blame for the deadly chemical attack on a Damascus suburb on August 21, which killed at least 1,400, including 400 children.
He said he expected to secure approval from Congress for military strikes which he insists will be "limited and proportionate" and will not involve US soldiers' "boots on the ground" in Syria. He has won the backing of key Republican figures, but the issue is dividing political opinion in the US.
There were clear signs of division at Prime Minister's Questions in the Commons, where Mr Cameron expressed his "regret" at Labour's stance in last week's vote when MPs rejected British involvement in any military action against Assad.
The Prime Minister repeated his assurance that the UK "can't be part and won't be part" of any strike on the regime after his shock defeat.
But he accused the Opposition of needlessly blocking even the principle of an armed response to the use of chemical weapons after it failed to accept his offer of a second vote to approve direct action.
Following measured exchanges on the need for increased humanitarian aid and engagement in a peace process for Syria, the debate turned acrimonious as Miliband said the vote was "not about Britain shirking its global responsibility, it was about preventing a rush to war".
The Labour leader said the key was now bringing other countries - including Iran - to the table to help negotiate a peaceful resolution to the conflict.
Cameron hit back: "Last week the House of Commons voted clearly and I have said I respect the outcome out of that vote and I won't be bringing back plans for British participation in military action.
"I agree with you that we must use everything we have in our power - our diplomatic networks, our influence with other countries, our membership of all the key bodies, the G8, the G20, the UN, the EU, Nato - we must use all that influence to bring to bear.
"My only regret of last week is that I don't think it was necessary to divide the House on a vote that would have led to a vote but he took the decision that it was."
The Commons' longest-serving MP, Father of the House Sir Peter Tapsell, asked Mr Cameron to answer "the Armageddon question... if the Americans illegally bombard the Assad forces, and Assad legally invites the Russians in to degrade the rebels, what will Nato do?".
The Prime Minister replied: "We would only support action that would be legal, we would only support action that was proportionate."
But he added: "If no action is taken following President Obama's red line and if no action is taken following this appalling use of chemical weapons, you have to ask yourself what sort of Armageddon are the Syrian people going to be facing?"
In response to a call from Labour MP Joan Ruddock to use the G20 to pursue peace talks, rather than US bombing of Syria, Mr Cameron said: "(Mr Obama) set a very clear red line that if there was large-scale chemical weapons use, something had to happen.
"We know that the regime used chemical weapons on at least 14 previous occasions. To ask the president of the United States, having set that red line and having made that warning, to step away from it, that would be a very perilous suggestion to make, because in response I think we would see more chemical weapons attacks from the regime."
Cameron said the world still had to take a "very tough response" to the use of chemical weapons.
That included aiding rebel forces, he said, so that President Assad was forced to recognise there was "no victory he can win against his own people" and to accept the need for a peace process leading to a transitional government.
Downing Street has refused to rule out seeking authorisation to supply weapons to rebels, saying only that no decision has been made.
The UK would also use its "diplomatic muscle" to press other nations to contribute more to aid for refugees, said Mr Cameron, after the UN said the number had passed two million. With a shortfall of around 50% in the donations required by the UN, the international community was "letting down" those suffering as a result of the two-year civil war.
One of the MPs who rebelled over Syria, Jesse Norman, has been removed from his role on the No 10 policy board.
A Downing Street source said: "Jesse has done some very good work on the policy board but he couldn't support the Government on a three-line whip the other day, so he has left that position."