BEIRUT: A long-expected Syrian army onslaught to drive rebel forces out of Aleppo is imminent, following a military build-up around the country's biggest city, a senior United Nations official said.

President Bashar al-Assad's forces killed more than 80 people in a series of attacks across the country on Friday and late on Thursday, opposition sources said, reporting intensified fighting in several cities.

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 has 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.

As fighting in Syria worsened, Moscow was reported to be sending three large landing ships to a Russian naval maintenance facility at the Syrian port of Tartus in the Mediterranean.

The naval ships, each carrying up to 120 marines, would reach Tartus in the next few days to resupply the base, Russian news agencies quoted a source in the General Staff as saying.

Russia, a close ally of Assad, had earlier said it was preparing to send marines to Syria in case it needed to protect personnel and remove equipment from the Tartus facility.

Russian airline Aeroflot announced two weeks ago that was stopping its flights to and from Damascus from Monday, citing “low demand”. The Foreign Ministry in Moscow has warned Russian citizens not to travel to Syria.

Annan resigned on Thursday, complaining of “finger-pointing” at the United Nations while the bloodshed in Syria went on.

Annan's mission, centred on a six-point peace plan and an April ceasefire that never took hold, had looked increasingly futile amid the violence in Damascus, Aleppo and elsewhere.

Annan suggested that the continued arming of all sides in the conflict and deadlock at the Security Council had undermined his ability to pursue a diplomatic solution.

“The increasing militarisation on the ground and the clear lack of unity in the Security Council, have fundamentally changed the circumstances for the effective exercise of my role,” Annan told reporters.

In an editorial published on the Financial Times website, Annan said Russia, China and Iran “must take concerted efforts to persuade Syria's leadership to change course and embrace a political transition” - meaning the departure of Assad.

“It is clear that President Bashar al-Assad must leave office,” Annan said.

Russian President Vladimir Putin, a strong supporter of Assad, said he regretted Annan's decision to step aside and referred to him as a “brilliant diplomat.”