Britain today condemned as "inexcusable and indefensible" the decision by Russia and China to veto a United Nations Security Council resolution threatening sanctions against the regime of President Bashar Assad in Syria.
The vetoes came just hours after Prime Minister David Cameron issued a direct plea to Russian President Vladimir Putin not to block action against Assad, warning that Syria faces civil war unless the dictator leaves.
"The alternative to political transition at the top of Syria is revolution from the bottom in Syria," warned Mr Cameron during a visit to Afghanistan.
As fighting continued for the fifth day on the streets of capital Damascus, foreign secretary William Hague accused Moscow and Beijing of turning their backs on the people of Syria "in their darkest hour", the Press Association reported.
Mr Hague said Britain would now back a proposal from peace envoy Kofi Annan for a final 30-day extension of the UN observer mission in Syria after its mandate runs out on Friday, in order to give the Assad regime "a last opportunity to live up to its commitments".
But the UK will also seek tighter EU sanctions and look at what more it can do to help the Syrian opposition, he said.
And he made clear that, while today's resolution would not have authorised military action, the door had not been closed on the use of force in future, saying that Britain "will rule nothing out" in its efforts to end the killing.
In a warning to those in positions of power in Damascus, the foreign secretary said: "Today's veto does nothing to change the fact that the Assad regime is doomed.
"My message to all those in the regime is that they will be held accountable for their actions.
"The pressure on them will not relent for an instant in spite of the veto today.
"We will do all that we can do to end the killing and will rule nothing out in the future in these efforts."
Today's developments in New York came a day after the most daring strike by rebels since the outset of the uprising 16 months ago, when a bomb killed three leading regime figures, including Assad's brother-in-law and the defence minister, attending a crisis meeting in Damascus.
Government forces used attack helicopters, heavy machine guns and mortar shells against rebels in a fifth day of fighting in Damascus, while thousands of refugees flooded across the border into Lebanon.
The Security Council voted 11-2 in favour of a resolution which would have created a trigger for global sanctions if Damascus failed to live up to commitments to end the use of heavy weapons against insurgents and civilians, to step back from violence, permit UN observers to operate unhindered and move towards a political resolution.
But China and Russia were able to block agreement by wielding the vetoes they hold as permanent members of the 15-nation Council.
They instead backed a rival Russian resolution which would extend the observer mission by up to 90 days but was dismissed by the UK as ineffective.
Mr Hague - who has just returned from a visit to refugee camps in Jordan housing thousands of Syrians who have fled the violence - said he was "appalled" by their refusal to back the action requested by Mr Annan.
Speaking at the Foreign Office in London, Mr Hague said: "In the view of the UK, the decision by Russia and China to veto this resolution is inexcusable and indefensible.
"When it came to the time to turn agreements which they have supported into action to end the violence, they stood aside.
"They have turned their back on the people of Syria in their darkest hour."
He added: "The Syrian refugees I met in Jordan this week were desperate and despairing.
"They will feel abandoned and betrayed by this unjustifiable veto.
"I believe Russia and China will pay a serious price in the Middle East diplomatically and politically for taking this position.
"Many observers will conclude that they have put national interest ahead of the lives and rights of millions of Syrian people.
"And they will be held increasingly responsible for worsening the crisis in Syria."
Mr Hague dismissed Russia and China's characterisation of today's resolution as a Western plan, pointing out it was called for by the Arab League and backed by non-permanent Security Council members including India, Morocco and Colombia.
"It was not a Western proposition - it was what people and governments all over the world wanted to see," he said.
"And there was nothing in it which would have authorised military action."
Shadow foreign secretary Douglas Alexander said: "I fear today's veto by Russia and China will be seen by Assad's regime as giving it the green light to continue with its brutal crackdown which has already cost the lives of so many innocent people.
"The conflict in Syria today symbolises not only a tragic betrayal of responsibility by the Syrian leadership, but a tragic and damaging divide within the international community."