International mediator Kofi Annan said he feared an "imminent battle" for Syria's biggest city Aleppo, Reuters reports.

Syrian opposition sources said helicopters from President Bashar al-Assad's military pounded a rebel-held part of the city on Saturday and armoured 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.

French President Francois Hollande said he would keep trying to convince Russia and China, which have Security Council vetoes, to support harder sanctions against Assad that they have opposed during the 16-month-old uprising.

"I will once more address Russia and China so that they recognise there would be chaos and civil war if Bashar al-Assad isn't soon stopped," said Hollande.

He said the Syrian government knew it was doomed and would use force until the very end, adding: "The role of the member states of the U.N. Security Council is to step in as quickly as possible."

Russia played down speculation that it might offer Assad asylum, with Foreign Minister Sergei Lavrov saying on Saturday Moscow had no such agreement and was not even thinking about it.

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.