SYRIAN forces killed 20 people in clashes with rebel fighters in a strategically important district of Aleppo, while the UN warned that an army onslaught in the city was imminent.

Government forces killed more than 110 people in attacks across the country yesterday and late on Thursday as fighting intensified in several cities and in Aleppo's Salaheddine district.

The violence came within hours of UN Secretary-General Kofi Annan quitting as peace envoy for Syria, blaming the failure of his peace mission on "finger-pointing" at the UN which had rendered impotent mediation efforts during the 17-month uprising against President Bashar al Assad.

"Fierce clashes are taking place in Salaheddine right now and there are 20 civilians killed," said one activist. "The district is being shelled by artillery and helicopters."

The Syrian army has reinforced its positions in and around Aleppo over the past two weeks, while conducting daily artillery and aerial bombardments of rebel forces in the city.

"The focus two weeks ago was on Damascus," said Herve Ladsous, UN Under-Secretary-General, in New York. "The focus is now on Aleppo, where we have reason to believe that the main battle is about to start."

For both rebels and the Government, as Syria's commercial hub with 2.5 million inhabitants, Aleppo is a prize that could determine who emerges dominant in a struggle that has already cost 18,000 lives.

Communication networks in Aleppo were cut for the third day, hampering attempts by rebels to co-ordinate and forcing them to use couriers to deliver orders.

Rebels in the city said they had lost 50 men in fighting in the three days up to Thursday. Rebel commander Abu Zaher said his forces had taken several officers prisoner and seized weapons after besieging a police station for days.

Syrian forces killed at least 50 people during clashes with rebels in the central city of Hama on Thursday, while a helicopter bombardment killed 16 near the southern town of Deraa, where the revolt first broke out.

In a Damascus suburb, 21 people were killed when mortar rounds landed in a Palestinian camp that holds 100,000 refugees.

Syrian troops backed by armour killed at least six people yesterday when they bombarded a southern district of the capital, where rebels have re-established a presence following a partial army withdrawal.

Meanwhile, Russia denied it was sending naval vessels to a Syrian port. The Russian defence ministry dismissed a report yesterday attributed to a source on the general staff that it was sending three landing ships with marines aboard to visit its facility at Tartus on the Syrian coast.

About 1000 Syrians, including a defecting brigadier-general, have fled to Turkey in the past 24 hours. The number of Syrian refugees in Turkey now stands at 45,500, among them at least 25 army generals.

Mr Annan's stepping down was described by Russian President Vladimir Putin, a strong supporter of Mr Assad, as a matter of regret, while his UN ambassador, Vitaly Churkin, blamed western powers in the UN Security Council for the failure of the UN's peace efforts.

The White House blamed Moscow and Beijing, which vetoed three resolutions intended to put the pressure on Mr Assad.

In an article published on the Financial Times website, Mr Annan said Russia, China and Iran "must make concerted efforts to persuade Syria's leadership to change course and embrace a political transition.

"It is clear that President Bashar al Assad must leave office," Mr Annan said.