World leaders have called on President Bashar Assad to stop the massacre of civilians in Aleppo, Syria's second city.
UN secretary-general Ban Ki-moon yesterday demanded a halt to the feared slaughter in Aleppo as government forces targeted the city.
UK foreign secretary William Hague warned of "a potential massacre" and said the worsening civil war could see the UK offer more help to rebel fighters battling to topple the regime.
Speaking in London, Mr Ban said: "I'm seriously concerned by the escalating violence.
"I urge the Syrian government to halt the offensive. The violence from both sides must stop."
Mr Hague accused Assad's men of launching "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 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, 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.
"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."
Activists say 19,000 people have been killed since February.
Mr Hague, who met Mr Ban in London before last night's Olympics opening ceremony, said the "dire situation" in Aleppo showed why the UN Security Council should have agreed a resolution aimed at raising the pressure on Assad.
The move crumbled when permanent members Russia and China vetoed the proposal -- despite previously backing special envoy Kofi Annan's peace plan for the region.
"With Aleppo clearly about to be attacked, every country has a responsibility to reinforce that message, with or without a resolution of the Security Council," said Mr Hague.
He spoke of his "extreme frustration" that the resolution failed, saying: "That would really have placed great pressure on the Syrian regime."
Mr Ban called on the five permanent Security Council members to show "strong leadership" and take "primary responsibility" for tackling the crisis.
He added: "We should show a sense of duty [and] solidarity in the name of humanity."