Foreign Minister William Hague has warned Syria is planning a massacre in Aleppo as UN Secretary General Ban Ki-Moon called on the Assad regime to stop its assault.
Mr Ban said: “I’m seriously concerned by the escalating violence in Aleppo.
“I urge the Syrian government to halt the offensive.
“The violence from both sides must stop for the sake of suffering civilians in Syria.”
The pair were speaking after a meeting to mark the Olympic Truce as the London prepared for last night’s Games’ opening ceremony.
Mr Hague spoke of his “deep concern” at reports Syrian forces had begun “a vicious assault” on Aleppo – and urged Russia and China to join criticism of the attack.
He said: “This utterly unacceptable escalation of the conflict could lead to a devastating loss of civilian life and a humanitarian disaster.
“It will add to the misery being endured by the Syrian people and plunge the country further into catastrophic civil war.
“The Assad regime must call off this assault. I call on all countries around the world, including the permanent members of the Security Council, to join us in condemning these latest actions and to insist on a political process to end the violence in Syria.
“All those with influence on the Syrian regime should bring it to bear now. No nation should stand silent while people in Aleppo are threatened with a potential massacre.”
The Foreign Secretary said the “dire situation” showed why the Security Council should have agreed last week’s resolution.
His meeting with Mr Ban came after Syria’s most prominent defector, Republican Guard commander Brigadier Gen Manaf Tlass, who fled Syria earlier this month, put himself forward as someone to unite the fractured opposition groups trying to topple Assad.
The factions met in Qatar yesterday to thrash out a deal over a transitional leadership.
Activists say 19,000 people have been killed since the uprising began last February as the Arab Spring pro-democracy uprisings swept the Middle East and North Africa.
Earlier this week, former army commander Colonel Richard Kemp, who led UK forces in Afghanistan, said the escalating civil war meant it was more likely western governments would intervene to stop the bloodshed spreading to neighbouring countries.
In a paper for the Royal United Services Institute, he said: “Western political leaders may have no appetite for deeper intervention.
“But as history has shown, we do not always choose which wars to fight – sometimes wars choose us.”
Mr Hague warned of “a potential massacre” in Aleppo and spoke of his “extreme frustration” at the failure to agree a UN resolution last week. “That would really have placed great pressure on the Syrian regime,” he said.
“It was the right thing to do and would have made a big difference if it had been adopted. We will continue in the UK to support every means of bringing about a peaceful resolution and transition in Syria.”
Prime Minister David Cameron also urged the Syrian regime to stop after talks with Turkish prime minister Recep Tayyip Erdogan. Mr Cameron said they discussed “the very real concerns we have that the regime is about to carry out some truly appalling acts in and around the city of Aleppo”.
“This would be completely unacceptable, this regime needs to recognise it is illegitimate, it’s wrong, it needs to stop what it is doing and the international pressure against this regime, against Assad, is only going to build until he finally goes,” he said.