Eight civilians also died in the latest violence, the Syrian Observatory for Human Rights said. Six of them, including a child, were killed in regime air strikes on the town of Jisr al-Shughur.
A car bombing killed 13 people in nearby Afrin, the city that Turkey-backed rebels seized last year from Kurdish fighters.
Syria’s civil war has killed a total of more than 370,000 people and spiralled into a complex conflict since starting in 2011 with the repression of anti-government protests.
Russian and regime aircraft have since late April ramped up deadly bombardment of the Idlib region of some three million people in northwest Syria, despite a deal to avert a massive government assault.
Regime forces have also been locked in battle with jihadists and allied rebels on the edges of the bastion held by Syria's former Al-Qaeda affiliate Hayat Tahrir al-Sham (HTS), including the north of Hama province.
Clashes raged since Wednesday night in northern Hama after a small advance by jihadist-led forces, the Syrian Observatory said.
Fighting and bombardment since the launch of the attack late Wednesday killed at least 57 regime forces and 44 jihadists and allied rebels, the Britain-based war monitor said, giving a revised death toll.
“The fighting is ongoing as regime planes and artillery pound the area,” Observatory head Rami Abdel Rahman said.
HTS spokesman Abu Khaled al-Shami said the jihadist and rebel fighters attacked after dark, taking control of the village of Hamameyat and a hilltop.
In air raids Thursday, a civilian was killed in a Russian strike on the Idlib town of Latamneh, the Observatory said, while rebel shelling cost the life of a woman in regime-held outside the jihadist stronghold.
Car bomb in Afrin
Elsewhere in Syria, eight civilians were among the 13 people killed in a car bomb near a checkpoint outside Afrin, the Observatory said.
Turkish troops and Syrian proxies took control of Afrin from Kurdish forces they consider “terrorists” in March last year after a two-month air and ground offensive.
Those killed also included four fighters.
“Among the victims, at least six are originally from Eastern Ghouta,” a former rebel bastion near Damascus retaken by the regime last year, Abdel Rahman said.
There was no immediate claim of responsibility for the blast, but a commander with a pro-Ankara faction accused Kurdish fighters.
Since the Turkish-backed rebel takeover, the UN and human rights groups have documented widespread abuses in Afrin.
The UN and Amnesty have also reported patterns of house appropriations by fighters and civilians bussed to Afrin during the surrender last year of Eastern Ghouta.
Half of the Kurdish enclave's 320,000 residents have fled, according to a report by the UN Commission of Inquiry, and most are unable to return.
Also on Thursday, several people were wounded in a car-bomb blast near a church in the Kurdish-majority city of Qamishli in northeast Syria, an AFP journalist and state media said.
The metal gate of the church was slightly dented by the blast, but the building otherwise emerged intact.
Endless rounds of peace talks
Endless rounds of UN-led peace talks have failed to stem the bloodshed in Syria.
UN peace envoy Geir Pedersen, however, is pushing ahead with a 17-month-old effort to form a committee to write a post-war constitution.
Both he and Damascus on Wednesday expressed “progress” towards forming the panel, whose composition has been the subject of dispute.
Shored up by a series of Russian-backed victories since 2015, the regime wants to amend the current constitution, but the opposition wants an entirely new one.
A September deal between Moscow and Ankara was supposed to avert a massive regime offensive on Idlib, but it was never fully implemented and HTS took full administrative control in January.
More than 570 civilians have been killed in regime and Russian air strikes on northwest Syria since late April, according to the Observatory.
Rebel fire during the same period has killed more than 45 civilians in adjacent government-held areas.
