World leaders tonight called on President Bashar Assad to stop the massacre of civilians in Syria's second city.
United Nations (UN) Secretary General Ban Ki-moon, flanked by Foreign Secretary William Hague, demanded a halt to the feared slaughter in Aleppo as government forces targeted the city.
Mr 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 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."
Mr Hague accused President Assad's men of launching "a vicious assault" on Aleppo - and urged Russia and China to join criticism of the attack.
The Foreign Secretary 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."
Activists say 19,000 people have been killed since the Syrian uprising began last February as the Arab Spring pro-democracy uprisings swept the Middle East and North Africa.
Mr Hague, who met with Mr Ban at the Foreign Secretary's official London residence ahead of tonight's Olympics opening ceremony to promote the Olympic Truce spirit, said the "dire situation" in Aleppo showed why the UN Security Council should have agreed a resolution last week aimed at raising the pressure on President 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.
"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."
Mr Hague said that included offering more help to rebel fighters, warning: "I want them to know in the Assad regime that the more they do this sort of thing the more we will increase the practical support we give to opposition movements in Syria."
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."
David Cameron also urged the Syrian regime to "stop what it is doing".
After talks with Turkish prime minister Recep Tayyip Erdogan, the Prime Minister 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".
Mr Cameron added: "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."
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