Activists said on Saturday that troops backed by tanks, artillery fire and helicopter gunships were advancing on southwestern districts of Aleppo, where rebel forces were mostly concentrated.

"You can say that the fightback has begun," the head of the Britain-based Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP news agency.

Rahman said the worst clashes since the start of the Syrian uprising in March 2011 were now taking place in several Aleppo neighborhoods.

Before this month, Aleppo had seen little fighting during the uprising, but Western powers fear it could now become a key battleground.

Government troops and heavy armor have been massing on the outskirts of the city for several days, fueling fears that what pro-government media have termed "the mother of all battles" could be about to start.

"Aleppo will be the last battle waged by the Syrian army to crush the terrorists and after that Syria will emerge from the crisis," the Al-Watan newspaper said.

The Syrian regime of President Bashar al-Assad is attempting to reassert its authority after opposition fighters took over much of Aleppo, a commercial hub, on July 20.

Assad has sent reinforcements to Aleppo to help retake the city.

Activists said 17 people were killed in the city on Friday from a toll of 160 across the country.

With the rebels outnumbered and outgunned, a major government offensive in Aleppo seems likely to cause heavy casualties.

International fears

United Nations Secretary General Ban Ki-moon has called on Damascus not to go through with its threatened offensive in Aleppo.

"I'm seriously concerned by the escalating violence in Aleppo. I urge the government to halt the offensive. The violence from both sides must stop for the sake of suffering civilians."

British Prime Minister David Cameron said there were "very real concerns that we have that the Syrian regime is about to carry out some truly appalling acts around and in the city of Aleppo."

Russia warned on Saturday of an imminent "tragedy" in Aleppo, but said it was unreasonable to expect that the Syrian regime would accept Aleppo being in rebel hands.

"How can you hope that in such a situation, the government will simply reconcile itself and say 'All right, I was wrong. Come on and topple me, change the regime'?" Russian Foreign Minister Sergei Lavrov told a news conference.

Russia has used its veto right on the UN Security Council to block sanctions against Syria, but denies it is backing Assad's regime.

Moscow says it is remaining impartial in the conflict, while accusing the West of siding with Syrian rebels.

Meanwhile, the former head of the UN observer mission in Syria, Major General Robert Mood, warned that the violent unrest in Syria might not end even if President Assad were to stand down in view of sectarian tensions in the country.

The many years of domination of the Sunni Muslim majority by Assad's Alawite minority has caused major rifts.

"Many think that if Bashar al-Assad falls or that if he is given an honorable exit ... the problem will be solved. That is an over-simplification one should be wary of," Mood said.

tj/ncy (dpa, AFP)