President Bashar al-Assad's forces killed more than 110 people in a series of attacks across the country on Friday and late on Thursday, opposition sources said, reporting intensified fighting in several cities including the Salaheddine district of Aleppo.

"Today fierce clashes are taking place in Salaheddine right now and there are 20 civilians killed. The district is being shelled by artillery and helicopters," an activist told Reuters by Skype.

The violence came within hours of former United Nations Secretary-General Kofi Annan quitting as international peace envoy for Syria and underlined the impotence of mediation efforts in the 17-month-old uprising against Assad.

The fighting spread to Aleppo from Damascus after a bomb attack on Assad's security headquarters in the capital on July 18, which killed four of the president's senior aides and encouraged rebels to step up hostilities.

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. The focus is now on Aleppo, where there has been a considerable build-up of military means, and where we have reason to believe that the main battle is about to start," Herve Ladsous, the Under-Secretary-General for Peacekeeping Operations, said in New York.

Russia, an ally of Syria, denied it was sending naval vessels to a Syrian port. The Defense Ministry dismissed a report attributed to a source on the general staff earlier on Friday that it was sending three landing ships with marines aboard to visit a Russian maintenance facility at Tartus.

HELICOPTER BOMBARDMENT

Opposition activists said 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 rebels near the southern town of Deraa, where the revolt first broke out.

In Damascus, at least 20 people were killed when security forces fired three mortar rounds at a Palestinian camp that is home to 100,000 refugees, medical sources said. Palestinian leaders in the West Bank and Gaza Strip condemned the attack.

Syrian troops backed by armor killed at least six people on Friday when they bombarded a southern district of the capital, where rebels have re-established a presence following a partial army withdrawal from the area, residents and opposition activists said.

In Aleppo, Syria's commercial hub, activists reported heavy clashes in the southwestern area of Salaheddine, which the army has repeatedly tried to wrest from the rebels. Rebels said they had lost 50 men in fighting there in the three days up to Thursday.

For both rebels and the government, Aleppo with its 2.5 million inhabitants is a prize that could determine which side emerges dominant in a struggle that has already cost 18,000 lives, according to the opposition.

Rebels said on Friday they had captured a large police station in Aleppo after days of clashes. Rebel commander Abu Zaher said they had taken several police officers prisoner and seized weapons and ammunition.

Internet and telephone networks in Aleppo were cut for the third day, hampering attempts by rebels to coordinate and forcing them to use couriers to deliver orders.

Rebel control in areas around Aleppo is by no means total, with some villages loyal to Assad and others favoring the opposition. Police and soldiers were setting up mobile check points on some main roads leading into the city from the north.

Soldiers were launching rockets at insurgents from time to time from an infantry school north of Aleppo, rebels said.

About 1,000 Syrians, including a defecting brigadier-general, have fled to Turkey in the past 24 hours to escape the intensifying violence, a Turkish official said on Friday.

The group brought the number of Syrian refugees in Turkey to 45,500. At least 25 Syrian generals have taken refuge in Turkey.