Government troops have fought back rebels near the airport of Aleppo, Syria's state media said, in the first official acknowledgement that combat had neared a strategic gateway to the country's largest city.
As fighting raged in both Aleppo and the capital Damascus, the United Nations announced that Lakhdar Brahimi, a former Algerian foreign minister and veteran UN diplomat, would serve as its new peace envoy to Syria, aiming to resume efforts for a diplomatic solution to the civil war.
Mr Brahimi, who previously served as envoy to Iraq and Afghanistan, replaces former UN secretary-general Kofi Annan, who announced he would leave the post by the end of this month after failing to bring about a ceasefire despite months of negotiations.
The announcement came as UN observers in Syria were beginning to pack up in preparation to close down their mission. Deployment of the observers was one of the only steps taken under Mr Annan's peace plan. The team was intended to watch over a ceasefire which never took hold, and so was left trying to chronicle some of the more egregious instances of bloodshed.
Both sides have "chosen the path of war", said the UN's assistant secretary-general for peacekeeping, Edmond Mulet. The UN plans to keep a small liaison office to support any future peace efforts.
The 17-month conflict between president Bashar Assad's regime and rebels trying to bring him down has left some 20,000 people dead, according to estimates by anti-Assad activists.
The escalating fight has in the past two months turned to battles in the country's two main cities, Damascus and Aleppo - once firm bastions of Mr Assad's rule. Rebels have managed to keep fighting in both cities despite facing overwhelming regime firepower.
In Damascus, activists reported heavy shelling and clashes in many areas on Friday, including western districts believed to have rebel pockets. Damascus-based activist Moaz al-Shami described the shelling as "non-stop" and said gunners were firing from the Qassioun mountains overlooking the city.
Regime forces have been fighting for nearly a month to crush the rebels' footholds in Aleppo, in the north of the country. Rebels have been driven from some areas, but the report of clashes near the airport suggests the battles could be shifting to new fronts.
Syria's official Sana news agency said "armed terrorist groups" - the regime's phrase for rebels - had been pushed out from areas on both sides of the airport, which is about nine miles south-east of Aleppo's historical centre.