UN Secretary General Ban Ki-moon has called on the Syrian army to stop its assault on the northern city of Aleppo.
Opposition activists say that Syrian troops and armour are amassing around Aleppo, which is the country's main commercial and industrial hub.
Speaking during a visit to London, Mr Ban called on Syrian President Bashar al-Assad to end the attack.
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."
British Foreign Secretary William Hague has warned of a massacre in the city.
Mr Hague spoke of his "deep concern" at reports Syrian forces had begun "a vicious assault" on Aleppo and he urged Russia and China to join criticism of the attack.
"This utterly unacceptable escalation of the conflict could lead to a devastating loss of civilian life and a humanitarian disaster," he said.
"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."
Mr Ban also said he was "deeply" concerned about reports of the possible use of chemical weapons by Syria, and demanded the government state it would not use them "under any circumstances".
He said: "I remain deeply concerned about the reports of the possible use of chemical weapons.
"I demand .... that the Syrian authorities categorically state that they will not use chemical or other weapons of mass destruction under any circumstances."
The men were speaking after a meeting to mark the Olympic Truce as London prepares for tonight's Olympic opening ceremony.
Meanwhile, British Prime Minister David Cameron also urged the Syrian regime to "stop what it is doing".
After talks with Turkish Prime Minister Recep Tayyip Erdogan, Mr Cameron said they had 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 added.
Elsewhere, a Syrian member of parliament who represents Aleppo has defected to Turkey.
Ikhlas al-Badawi is the first member of the assembly elected in May and dominated by President Bashar al-Assad's Baath Party to defect.
Ms al-Badawi said: "I have crossed to Turkey and defected from this tyrannical regime ... because of the repression and savage torture against a nation demanding the minimum of rights."
Ms al-Badawi, a Sunni Muslim, was chosen by state authorities to run for parliament on behalf of the "labourers and peasants" sector.
Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.