Air strikes by Syrian government ally Russia killed 78 fighters from a Turkey-backed rebel group in northwest Syria on Monday, a Britain-based war monitor said.
Russian warplanes also wounded 100 people when they targeted a training camp of the Faylaq al-Sham faction in the Jabal Duwayli area in Idlib province, the Syrian Observatory for Human Rights said.
In March, a fragile truce brokered between Moscow and Ankara stemmed a deadly months-long Russia-backed regime military offensive on the country's last major rebel stronghold in Idlib that displaced almost a million people from their homes.
The National Liberation Front, an umbrella group of Ankara-backed rebels, told AFP that Russian strikes on Monday had hit one of its positions and caused casualties, but did not give an exact death toll.
NLF spokesman Sayf Raad denounced the "Russian aircraft and regime forces continuously violating the Turkish-Russian deal in targeting military positions, villages and towns".
The March truce has largely held with the exception of some bombardment and intermittent air strikes on the area, including a US drone strike on Thursday that killed 17 jihadists, according to the Observatory.
Hayat Tahrir al-Sham, led by Syria's former Al-Qaeda affiliate, and allied rebels dominate the region of some three million people, many living in camps after being displaced by the nine-year war.
After a string of military victories backed by Russia, the Syrian government has regained control of around 70 percent of the country, the Observatory says.
The war, which broke out after the brutal suppression of anti-government protests in 2011, has killed more than 380,000 people and displaced millions from their homes.