Syrian troops backed by helicopter gunships clashed with rebels near a barracks in Aleppo on Friday as battles broke out around a military airport elsewhere in the northern province, monitors said.
In Damascus, state news agency Sana said the army unearthed the bodies of 25 people shot execution-style in the Qadam district and blamed "armed terrorist groups," the regime's term for rebels.
In other developments, a masked gunman on a motorbike killed prominent Kurdish activist Mahmoud Wali on Thursday in northeastern Syria, fellow activists said.
And a tolerated opposition group said two members -- Abdel Aziz Khayer and Iyas Ayash -- had gone missing along with the man who had collected them from Damascus airport after a trip to China to discuss an end to the violence.
The National Coordination Committee for Democratic Change groups Arab nationalists, Kurds and socialists.
In Aleppo's Arkoub district, fighting erupted overnight near the Hanano army barracks, the Syrian Observatory for Human Rights said.
Several districts of the northern metropolis, including Sakhur in the northeast and Bustan al-Qasr in the centre, came under attack, the Britain-based monitoring group said.
Elsewhere in the province, fighting broke out between troops and rebels near the Meng military airport, it said.
Military airfields have been a key rebel target because the regime is increasingly using air power to launch devastating strikes.
Northwest of the capital, the Observatory reported a massive explosion believed to have been a car bomb. Heavy gunfire was heard afterwards but there were no immediate reports of casualties.
In the central province of Homs, a civilian was killed in dawn shelling of Rastan, while the eastern city of Deir Ezzor and the town of Daal in the southern province of Daraa were also bombarded.
In Damascus, Sana said, soldiers acting on a tip-off from local residents found a mass grave containing 25 bodies with their hands tied and eyes masked. They had been kidnapped and killed by rebels, it said.
But as the violence rages unabated, a top Nato general said in Brussels that the alliance does not believe that military intervention would bring any improvement in Syria's security situation.
Germany's Manfred Lange, Chief of Staff of Supreme Headquarters Allied Powers Europe, said the "political process has to be pushed forward, sanctions need to take effect. At the moment, this situation cannot be solved by the military in a responsible way."
UN Secretary General Ban Ki-moon held talks with his Arab League counterpart Nabil al-Arabi ahead of the UN General Assembly next week when Syria is expected to dominate speeches by world leaders.
"They discussed first and foremost the situation in Syria, with its political impasse, widespread human rights abuses, and growing humanitarian crisis," UN spokesman Martin Nesirky said of the talks.