Regime and Russian air strikes and shelling have killed at least 28 people including seven civilians in north-west Syria despite a ceasefire announced by Moscow, a war monitor said on Friday.
The attacks took place in the south of Idlib province and the north of Hama province on Thursday, the Syrian Observatory for Human Rights said.
The Idlib region of some three million people is supposed to be protected from a massive regime offensive by a buffer zone deal that Russia and Turkey signed in September.
But it was never fully implemented, as hardline Islamist fighters refused to withdraw from a planned demilitarised zone.
In January, the Hayat Tahrir Al Sham alliance led by Syria's former Al Qaeda affiliate extended its administrative control over the region, which includes most of Idlib province as well as adjacent slivers of Latakia, Hama and Aleppo provinces.
The Syrian government and and its ally Russia have intensified their bombardment of the region since late April, killing more than 360 civilians, according to the Observatory.
The latest attacks come despite Russia's announcement of a renewed ceasefire agreement with Turkey to take effect from Thursday. Turkish Foreign Minister Mevlut Cavusoglu said there were "serious and sincere efforts" with Moscow to stop the violence, but a full ceasefire had not been realised.
On Friday, Mr Cavusoglu said Turkey did not accept Russia's "excuse" that Syria would not listen to Moscow and stop attacks in Idlib.
"In Syria, who are the regime's guarantors? Russia and Iran," he told state news agency Anadolu in a televised interview. "Thus we do not accept the excuse that 'we cannot make ourselves listened to by the regime'."
Despite being on opposing sides of the Syrian war, Turkey and Russia have worked closely to find a political solution under the Astana process which also involves Iran, Syrian President Bashar Al Assad's other main ally. Turkey, as a backer of opposition forces, is the guarantor for Syrian rebels.
Mr Cavusoglu said that there had been "no issues" from the moderate opposition, and accused Damascus of send armed radical groups to Idlib from Aleppo, East Ghouta and Hama.
"We knew in the future after other areas had been captured, the regime would attack Idlib using the excuse of the radical groups' presence after sending them there," he said.
Syria's war has killed more than 370,000 people and displaced millions since it started in 2011 with the repression of anti-government protests.
Russia launched a military intervention in support of the regime in 2015, helping its forces reclaim large parts of the country from opposition fighters and jihadists.