International mediator Kofi Annan said he feared an "imminent battle" for Syria's biggest city Aleppo. Syrian opposition sources said helicopters from President Bashar al-Assad's military pounded a rebel-held part of the city on Saturday and armored units were positioned for an onslaught that could determine its fate.

"I am concerned about reports of the concentration of troops and heavy weapons around Aleppo, in anticipation of an imminent battle," Annan said in a statement. "The escalation of the military build-up in Aleppo and the surrounding area is further evidence of the need for the international community to come together to persuade the parties that only a political transition, leading to a political settlement, will resolve this crisis."

But a Syrian opposition leader urged foreign allies to circumvent the divided U.N. Security Council and intervene. "Our friends and allies will bear responsibility for what is happening in Aleppo if they do not move soon," said Abdelbasset Sida, the head of the Syrian National Council which is the main umbrella group for opposition to Assad. "Any action has to be from outside the Security Council through an Arab League initiative and through a resolution passed by the General Assembly," he said early on Sunday on a visit to the United Arab Emirates for talks with officials.

The Syrian Observatory for Human Rights, an opposition monitoring group, reported helicopter attacks on Aleppo's central Salaheddine district and fighting elsewhere in the city. One opposition activist said he had seen tanks and armored troop carriers heading for the district. (Reuters)