Syrian troops backed by helicopter gunships clashed with rebels near an army barracks in Aleppo, according to the Syrian Observatory for Human Rights (SOHR).
Fighting erupted overnight near the Hanano barracks in the Arkoub district of northeast Aleppo on Friday, the UK-based watchdog group said. Several districts of Aleppo, including Sakhur in the northeast and Bustan al-Qasr in the centre, came under overnight attack, SOHR said.
Elsewhere in Aleppo, fighting broke out between troops and rebels near the Meng military airport, SOHR said.
Military airports have been a key target for the rebels as the army has increasingly deployed fighter jets and helicopter gunships to launch devastating attacks against them.
SOHR further reported a massive explosion, believed to be a car bomb, northwest of Damascus. Heavy gunfire was heard afterwards but there were no immediate reports of casualties, it said.
Elsewhere in Damascus, the National Coordination Body (NCB) said that security forces seized three of its members shortly after they returned from an official trip to China, a spokesman for the group said on Friday.
NCB spokesman Khalaf Dahowd said NCB foreign affairs head Abdel Aziz al-Khair and executive committee member Eyas Ayyash arrived in Damascus on Thursday night and were followed by Syrian security agents to their car where they joined NCB member Maher Tahan.
"The car never made it to Damascus," Dahowd said. "They were kidnapped and we haven't heard from them since."
The men had returned from a visit to China in which they met Chinese Foreign Minister Yang Jiechi. China has urged Assad to talk to the opposition but has also defended the government, most notably by blocking diplomatic efforts to put pressure on Syria at the UN Security Council.
Five other members of NCB were reportedly detained by Syrian security agents on Monday.
In the central province of Homs, a civilian was killed in dawn shelling of Rastan, while the eastern city of Deir Az-Zor and the town of Daal, in the southern province of Deraa, also came under bombardment.
The violence across the country came a day after dozens of people were killed when an air raid hit a fuel station in the northern province of al-Riqqa on Thursday.
Al Jazeera's Andrew Simmons, reporting from Antakya in neighbouring Turkey on Friday, said there is "no doubt that it was a targeted attack on an area being used by civilians".
"The petrol station was crowded with vehicles," he said. "It was the only petrol station in the entire region that was open to the public, according to activists."
Simmons added that according to unconfirmed reports, the device used is known as a "barrel bomb."
"These sorts of things have been described in Aleppo before. It was devastating," he said.
"The casualties number in the dozens, and now we are getting unconfirmed reports that the death toll has reached 60."
In another development, diplomats from more than 60 nations and the Arab League met in The Hague, Netherlands, on Thursday to toughen and improve co-ordination of sanctions against Assad's regime.
"We need vigorous implementation," Uri Rosenthal, Netherlands foreign minister, told the opening of the Friends of Syria working group.
"Sanctions will only have an impact if they are carried out effectively. That is how we can make a difference."
The Friends of Syria group has already held three meetings at ministerial level in Tunis, Istanbul and Paris. Another is planned in Morocco in October and another later in Italy.
Source: Al Jazeera and agencies