Syrian government forces launched an offensive Tuesday to retake areas in the central Hama province lost in recent days to a sustained attack by rebel forces, including hardline Islamic factions.
The Britain-based Syrian Observatory for Human Rights said government helicopters carried out strikes on the villages of Maarkba, Lahia and al-Massina in the northern Hama countryside.
The rebel advance had been going largely unchecked this week, forcing government ground forces to retreat and cede villages along a strategic axis.
The area has been witnessing heavy shelling since early Tuesday and tit-for-tat attacks are continuing between the rebels and the government's forces, the observatory said.
Activist Abu Mohammed al-Hamoui said the rebels managed on Monday to take the village of Helfaya in a surprise attack. Regime forces retreated to a Christian area, Mahrada, which remains in the government's hands.
Jund al-Aqsa, a hardline jihadist group that has Syrian and foreign fighters, is the main force leading the attack on the rebel side. More moderate fighters are also taking part.
The rebel attack is named Marwan Hadid, after a Muslim Brotherhood-affiliated figure who died in a Syrian prison in the 1970s, highlighting the longstanding grievances of the Syrian opposition.
The observatory said that at least 20 rebel fighters have died in the clashes as well as dozens from the regime forces and their allies.
At least four people were killed and 23 others were wounded when planes raided the area of Maaret Misreen in the northern countryside of Idlib, in northwestern Syria, the Observatory said.
Meanwhile, south of Hama, the Syrian opposition voiced fresh concerns about rebel-held al-Waer, a suburb of Homs which has been under siege by government forces for three years. Tens of thousands of people live in al-Waer.
"Attacks have significantly escalated on the neighborhood in the past few days with mortar rounds, napalm-filled bombs, rockets, heavy artillery and high explosive cylinders hitting the neighborhood," warned Anas Abdah, head of the opposition Syrian National Coalition.
Abdah called for the international community to ensure aid could reach the besieged residents.
Opposition concerns about areas under siege have picked up again after the Daraya suburb of Damascus, which had been largely under a tight blockade since 2012, was evacuated.
Rebel fighters were evicted to opposition-held Idlib province and civilians were taken to government-controlled areas, in a hugely symbolic capitulation to President Bashar al-Assad's forces.
A local ceasefire agreement between Al-Waer residents and the al-Assad regime collapsed in March.
In 2015, 3,200 rebel fighters evacuated the neighbourhood as part of a deal. In return, trucks loaded with medical and food supplies entered from government-held areas.