Leaders of the opposition training camp were among those killed in an air strike in Jabal al Dweila, according to a Syrian opposition spokesperson.
Russian air strikes have targeted a training camp in northwestern Syria, opposition groups said.
Youssef Hammoud, a spokesperson for the Syrian opposition groups, said air strikes on Monday in the northwestern part of Idlib province, the last opposition enclave in Syria, targeted a military training camp for Failaq al Sham, one of the largest Turkey-backed opposition groups in Syria.
Leaders of the camp were among those killed in an air strike in Jabal al Dweila, according to Hammoud. The camp is close to the borders with Turkey.
Hammoud didn't give the death toll but some reports say dozens have been killed by the air strikes.
The National Liberation Front, an umbrella group of Ankara-backed opposition, also confirmed air strikes.
NLF spokesperson Sayf Raad denounced the "Russian aircraft and regime forces continuously violating the Turkish-Russian deal in targeting military positions, villages and towns."
Opposition vows to retaliate
“The factions of the National Front for Liberation we will respond to these violations,” said Naji al Mustafa, another spokesperson for the Turkish-backed forces, threatening to target government and Russian posts.
He called it a “crime” by Russia.
Turkey and Russia had brokered a truce in Idlib earlier this year to halt a regime offensive that displaced hundreds of thousands. But the truce remained shaky.
Turkey has long supported Syrian opposition forces in Syria. Russia has negotiated with Ankara to deploy observation teams in the opposition enclave to monitor the truce.
In March, a fragile truce brokered between Moscow and Ankara stemmed a deadly monthslong Russia-backed regime military offensive on the country's last major opposition stronghold in Idlib that displaced almost a million people from their homes.
Idlib is home of some four million civilians, many living in camps after being displaced by the nine-year war. Hayat Tahrir al Sham, linked to Al Qaeda terror group also has a presence in the region.
The war, which broke out after the brutal suppression of anti-regime protests in 2011, has killed more than 380,000 people and displaced millions from their homes.