Tanks and helicopters pounded a rebel stronghold in the Syrian city of Aleppo Saturday, signaling the start of a government offensive, as the opposition and international envoy Kofi Annan warned the international community of dire consequences.

Annan said he feared an "imminent battle" for Syria's biggest city while the opposition warned its allies they would bear responsibility for a "massacre" if they don't act soon.

"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, Reuters reported.

"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."

The Syrian opposition, meanwhile, urged its foreign allies to intervene.

"This regime is planning for a big massacre in Aleppo," said Abdelbasset Sida, the head of the Syrian National Council, on a visit to the United Arab Emirates Sunday.

"Our friends and allies will bear responsibility for what is happening in Aleppo if they do not move soon," he said.

"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 added.

"The rebels now are fighting with primitive types of weapons against the killing machine. We need weapons that will allow us to stop tanks and planes. This is what we need," he said.

Fighting left at least 160 people dead across Syria on Saturday, opposition Local coordination Committees (LCC) said. "The heaviest clashes since the beginning of the revolution are taking place now in a number of neighborhoods in Aleppo city," the LCC said.

The death toll so far, according to LLC, is more than 16,000, while the U.N. estimate puts the deaths at 10,000.

French President Francois Hollande said he will continue efforts to convince Russian and China, which have vetoed the UN Security Council resolutions to intervene in Syria.

"I will once more address Russia and China so that they recognize there would be chaos and civil war if Bashar al-Assad isn't soon stopped," said Hollande. "The role of the member states of the UN Security Council is to step in as quickly as possible."

The Arab League has offered a "safe exit" for Syrian President Bashar al-Assad and his family if he quickly resigns and leaves the country, in the latest attempt from the international community to end 17 months of bloodshed in Syria.

Moscow, however, reiterated its earlier stance that Russia was not planning to provide asylum to Assad and denied that Russian President Vladimir Putin and his U.S. counterpart Barack Obama had discussed a potential destination for Assad if he decided to step down.