UN-Arab League envoy Lakhdar Brahimi says he hopes to avoid pitfalls encountered by Kofi Annan, who quit in frustration.
Syrian forces have launched new air strikes and shelled rebel strongholds in several key cities, particularly in and around the key northern commercial hub of Aleppo.
Warplanes on Saturday bombed the town of Azaz near Aleppo and shelling continued in the city’s neighbourhoods of al-Fardos, al-Sukkari, Bustan al-Zahra and Kallasa.
Fighting between government forces and rebels was also reported near Saad al-Allah al-Jabri’s Square in Aleppo’s centre and in the southern rebel stronghold of Salaheddin. Aleppo has become a main focus of the conflict since late July.
State television said soldiers “cleared terrorists and mercenaries” from the western district of Saif al-Sawla on Saturday.
In Damascus, fighting broke out in the heavily populated southern district of Tadamon, showing that the rebels still have pockets of resistance in the capital despite government forces last month claiming they had retaken it.
Meanwhile, in the eastern province of Deir al-Zor, activists said shelling by government forces continued on the city of Abu Kamal and the towns of Abou Hamman and al-Kashkiyya, and that armoured vehicles were seen moving into the eastern town of Mayadeen.
Government forces also pounded rebel-held areas of the central city of Homs and the southern city of Herak, activists said.
Government forces appear to be resorting to more attacks from the air against the more poorly armed rebel groups, while accounts of people being shot dead by snipers are increasing.
The violence come as the UN said veteran Algerian diplomat Lakhdar Brahimi would take over as international envoy from Kofi Annan, who quit this month after the failure of his peace plan.
Brahimi said the task of finding a solution to end the 17-month-old conflict, which activists say has killed 23,000 people, required “a lot of determination”.
His appointment was announced the day after the UN called time on its observer mission in Syria amid the escalating violence.
UN observers will depart from Syria on Sunday, but will leave behind a “liaison office” in Damascus, though its size and role have not been finalised, a UN spokesperson said.
The conflict in Syria has created an increasingly precarious humanitarian situation, triggering a major exodus of refugees that the UN said on Friday had risen to least 170,000, many of them fleeing to Turkey.
Laurent Fabius, the French foreign minister, on Friday called for President Bashar al-Assad’s regime to be “smashed fast” as he visited the largest of the refugee camps in Turkey.
“After hearing the refugees and their account of the massacres of the regime, Mr Bashar al-Assad doesn’t deserve to be on this earth,” Fabius said.
Assad’s regime has faced a string of high-level defections, including Prime Minister Riad Hijab and senior general Manaf Tlass. A bomb attack also killed four top security chiefs.
On Friday, reports emerged of the defection of Farouq al-Sharaa, Syria’s vice president, a claim Syrian state media denied.
Citing a statement from his office, Syrian TV said on Saturday: “Mr Sharaa has never thought about leaving the country or going anywhere.” Sharaa himself was not seen.
The rebel Free Syrian Army said in a statement that Sharaa, 73, who has been vice president since 2006, had tried to defect to the opposition.
“Initial reports show that there was an attempted defection, but that it failed,” the FSA’s military council said.
Sharaa a Sunni Muslim figure in the minority Alawite-led regime and has served in top posts for almost 30 years.
Meanwhile, in neighbouring Lebanon, three Syrians who had been kidnapped by armed gunmen in the capital Beirut on Saturday were released, the state NNA news agency reported.
The abductions come days after the Meqdad clan kidnapped 20 Syrians in Lebanon in retaliation against what they say was the abduction of one of their family members by a Syrian rebel group last week.
Two Turks have also been abducted, at least one of them by the Meqdad clan.
The wave of kidnapping has ratcheted up tensions in Lebanon and caused several Gulf countries to tell their nationals to leave. The United States and Turkey have also warned their nationals against all but essential travel to the country.