Russian warplanes also wounded 100 people when they targeted a training camp of the Faylaq al-Sham group in the Jabal Duwayli area in Idlib province, the Syrian Observatory for Human Rights said.
The so-called 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 what it called the "Russian aircraft and regime forces continuously violating the Turkish-Russian deal in targeting military positions, villages and towns".
The foreign-backed militancy in Syria, which broke out in 2011, has killed more than 380,000 people and displaced millions from their homes.
After a string of military victories backed by Russia, the Syrian government has regained control of around 70 percent of the country.