At least 44 regime loyalists, 39 rebel and militant fighters have been killed since Thursday on the edge of rebel-controlled Idlib, Syrian Observatory for Human Rights says.
Fierce clashes between regime forces and rebels have left 83 fighters dead in northwestern Syria in the past 24 hours, a Britain-based war monitor said on Friday.
The clashes on the edge of the rebel-controlled Idlib region have killed 44 regime loyalists and 39 rebels and militants since Thursday, the Syrian Observatory for Human Rights said.
The region of some three million people – almost half of them displaced from other parts of the country – is dominated by the Hayat Tahrir al Sham alliance led by Al Qaeda's former Syrian affiliate.
It administers a region that spans most of Idlib province as well as adjacent slivers of the neighbouring Latakia, Aleppo and Hama provinces.
Late on Thursday, the rebels and opposition allies launched a counterattack against regime forces in the northwest of Hama province.
They have since seized the villages of Tal Maleh and Jibeen there, the Observatory said.
"Violent clashes are ongoing, accompanied by regime and Russian air strikes," Observatory chief Rami Abdel Rahman said.
The fighting is raging near Christian and Alawite areas under regime control, he said.
Idlib is supposed to be protected from a massive regime offensive by a September buffer zone deal.
But the regime and its Russian ally have upped their bombardment of the region since late April, killing more than 300 civilians, according to the Observatory.
Opposition launches offensive in Syria's Hama province pic.twitter.com/GMdnXcLWxw— TRT World Now (@TRTWorldNow) June 7, 2019
UN warns of 'humanitarian catastrophe'
Also on Thursday, a senior UN adviser warned of a humanitarian catastrophe in Idlib, northwestern Syria, where Bashar al Assad's regime has been stepping up air strikes.
In a statement, Najat Rochdi, a senior humanitarian adviser to the UN Special Envoy for Syria, said three million people in Idlib need protection.
"A humanitarian catastrophe is a grave danger if the violence does not cease," Rochdi said.
"Attacks and fighting are also impacting civilians in government-controlled areas."
The statement came after a weekly Humanitarian Task Force meeting, during which she said she heard firsthand reporting from her UN colleagues based in the region.
The "increasingly horrific brutality in recent weeks ... has caused significant civilian causalities and the displacement of hundreds of thousands of people," she said.
Buffer zone truce violated
Turkey and Russia agreed last September to turn Idlib into a de-escalation zone in which acts of aggression would be expressly prohibited.
The Syrian regime, however, has consistently broken the terms of the truce, launching frequent attacks inside the zone.
"Let me be clear," said Rochdi. "The protection of civilians is paramount."
Rochdi called on warring parties to uphold their obligations under international law and immediately stop attacks on civilians and civilian infrastructure.
"We have a collective responsibility to the victims of this conflict, many of whom are too young to try to make sense of this senseless war," she said.
Eight years into Syria's civil war, the Idlib region is the last to remain beyond regime control apart from a large northeastern swathe held by the country's Kurds.
The war, which started in 2011 with the brutal repression of anti-regime demonstrations, has killed more than 400,000 people and displaced millions.