After UN Security Council paralysis on Syria forced peace envoy Kofi Annan to resign last week, and with his ceasefire plan a distant memory, rebels have been battered by the government onslaught in Aleppo and the capital Damascus.

A Reuters correspondent in Aleppo witnessed fierce street fighting in the Salaheddine district, a gateway into the city of 2.5million people.

Tanks pounded alleyways where rebels sought cover and one shell hit a building next to the reporter, pouring rubble onto the street and sending huge billows of smoke into the sky.

State television said Assad's forces were "cleansing the terrorist filth" from the country, which has been sucked into an increasingly sectarian conflict that has killed some 18000 people and could spill into neighbouring states.

In Damascus, jets bombarded the capital on Saturday as troops kept up an offensive they began a day earlier against the last rebel bastion there, a resident said.