Syrian rebels attacked a key army base in the northwest province of Idlib on Friday, the last regime bastion in the region, and regime warplanes launched air raids in Damascus province, a watchdog said.
The Syrian Observatory for Human Rights said several rebel groups pounded Wadi Deif amid violent clashes on the ground while regime warplanes launched air strikes around the army base.
Two rebel fighters were killed, the Britain-based Observatory said.
Rebels on the ground said the jihadist group the Al-Nusra Front was leading the offensive.
Syria's rebels have long aimed to take the base "no matter what losses they incur because of its strategic importance," the Observatory said in a statement.
It added that regime troops were deployed to protect the base, whose loss would cut the main supply route for reinforcements from Damascus to Syria's second city in the north, Aleppo.
Insurgents took a key base in October in the town of Maaret al-Numan, close to Wadi Deif, and right on the Damascus-Aleppo highway.
Army reprisals to retake the town failed.
Elsewhere, Al-Nusra and rebel fighters attacked regime troops guarding Aleppo airport, the Observatory reported, as clashes shook several neighbourhoods in the city, including around a military compound besieged by rebels.
In Damascus province, warplanes "for the first time attacked the Assal al-Ward area in the Kalamun region, killing one civilian, wounding dozens and destroying several homes," the Observatory said.
The military on Thursday withdrew from several areas in the province but further air raids were expected as regime forces sought to take back areas they had lost, the watchdog said.
It also reported fighting and bombardment in several districts of the capital itself overnight, with rockets hitting the Qaboon district in the northeast of the city and clashes between rebels and the army in the southern neighbourhood of Qadam.
Fighting took place in Daraya in southern Damascus which President Bashar al-Assad's forces have been trying to retake for weeks, it said.
There were also clashes in Yalda in the south and Douma in the northeast, as well as an attack on a military position northeast of Damascus between the provincial town of Irbin and the suburb of Harasta.
A sniper shot dead a man in a Palestinian refugee camp in Daraa in the south, and fighting was reported in areas near the border with Jordan, the watchdog said.
Several districts of Deir Ezzor city in the east were also bombarded, said the Observatory, which relies on a countrywide network of activists and medics in compiling its information and tolls.
Overnight shelling was reported in the central provinces of Hama and Homs.
On Friday at least 121 people were killed in violence across Syria including 43 civilians, said the Observatory which has reported a death toll of more than 45,000 since protests against Assad that erupted in mid-March 2011 became an armed rebellion.