Syria's ambassador in London was today summoned to the Foreign Office for rebuke over the "utterly unacceptable" violence being inflicted on civilians in the country as President Bashar Assad's regime cracks down on dissent.
Meanwhile, Foreign Secretary William Hague recalled Britain's ambassador in Damascus, Simon Collis, to London for "consultations" on what he termed an "utterly unacceptable situation which demands a united international response".
In a statement to the House of Commons, Mr Hague vowed to continue efforts to put pressure on Damascus, including by EU sanctions, and condemned as "a betrayal of the Syrian people" the decision by Russia and China to veto a United Nations resolution designed to end the bloodshed.
Saturday's veto - described as "incomprehensible and inexcusable" by Downing Street - has been followed by a fresh wave of assaults on rebel strongholds, which today saw heavy artillery pounding the city of Homs.
Activists said that at least 23 people were killed when shells struck a makeshift medical clinic and residential areas in the third day of a new offensive on Homs. Another 10 people were reported killed elsewhere.
In Cairo, Arab League secretary general Nabil Elaraby, who has led efforts to seek an international solution to the crisis, said he was "extremely alarmed and concerned" at the use of heavy weapons by government forces. But the regime denied shelling the city, blaming "armed terrorist groups" for attacking civilians and police.
The US has closed down its embassy in Damascus and evacuated all diplomats, amid fears that Assad is planning to unleash even greater violence to end an 11-month uprising which has claimed around 6,000 lives.
US President Barack Obama said: "We have been relentless in sending a message that it is time for Assad to go. This is not going to be a matter of if, it's going to be a matter of when."
Mr Hague left no doubt that he blames the Assad government for the "appalling" bloodshed and repression and said the regime was "doomed" because of its actions. He demanded assurances from Damascus that it would continue to ensure the protection of diplomats and staff at Britain's embassy, which remains in operation.
"The Syrian regime has deployed snipers, tanks, artillery and mortars against civilian protesters and population centres, particularly in the cities of Homs, Idlib, Hama and Deraa," Mr Hague told MPs.
"Thousands of Syrians have endured imprisonment, torture and sexual violence - including instances of the alleged rape of children - and the humanitarian position is deteriorating.
"The human suffering in Syria is already unimaginable and is in grave danger of escalating further.
"The position taken by Russia and China has regrettably made this more likely.
"However this Government, this House, our country and our allies will not forget the people of Syria. We will redouble our efforts to put pressure on this appalling regime and to stop this indefensible violence."
Mr Hague condemned the vetoes as "a grave error of judgment" by the Russian and Chinese governments, which would increase the likelihood of civil war in the strategically significant Middle Eastern state.
He told MPs: "The Syrian regime may have drawn comfort from events at the UN Security Council, but we will do everything that we can to make sure that comfort is short-lived.
"This is a doomed regime as well as a murdering regime. There is no way it can recover its credibility internationally or with its own people.
"The UN Security Council's failure to agree a resolution does not signal the end of our efforts to end the violence in Syria."
In a phone conversation, Mr Hague offered Mr Elaraby the UK's "complete support" for the Arab League actions to find a resolution in Syria, including the appointment of a special envoy.
The UK will intensify contacts with the Syrian opposition and will be "highly active" in supporting a new Arab-led group of Friends of Syria to co-ordinate diplomatic and economic pressure on the regime, Mr Hague told MPs.
As well as continuing to raise the issue of Syria at the UN, Britain will back a fresh round of EU sanctions at a meeting of foreign ministers on February 27.
And it will back the approval of "the strongest possible mandate" at the UN Human Rights Council summit in Geneva in March to ensure that those responsible for violations in Syria know that "there will be a day of reckoning and that they will be held to account".