BEIRUT — Russian war planes struck a camp of Turkish-backed rebel fighters in northwestern Syria on Monday, killing dozens of Syrian fighters as well as civilians harvesting olives nearby, according to a rebel spokesman, in the bloodiest surge in violence since a cease-fire came into effect seven months ago.

The strike targeted the group Faylaq al-Sham, whose base is near several refugee camps in the heavily populated province of Idlib. Several million people live in the province, many of them displaced from other parts of Syria by the country’s nine-year civil war.

Early reports put the toll at 34 dead and 70 wounded, but the death toll quickly rose to 60 with news of civilians killed outside the camp.

The fighters’ camp was at Kafr Takharim, near the Turkish border. The attack was the most violent breach of a cease-fire agreement that Russia and Turkey reached in March. Russia has occasionally made smaller strikes on militant groups, but such a large strike so close to the Turkish border is unusual.

Russia mounted a brutal offensive with the Syrian government last winter to retake Idlib province, the last rebel enclave in Syria. Turkey intervened, deploying thousands of its own troops to support the rebel fighters on the ground and protect civilians who started streaming toward the border.

The cease-fire has held since March, although Russia has repeatedly complained about the presence of rebel groups.

“Attacking the HQ of Faylaq al-Sham is nothing ordinary,” Omer Ozkizilcik, a researcher at SETA Foundation in the Turkish capital, Ankara, said in a Twitter post. “Turkey pays all Turkey-backed rebels in Idlib via Faylaq. The group is responsible for protecting the Turkish presence in Idlib and an essential part of the Turkish-Russian cease-fire agreement.”

Some connected the Russian airstrikes to Turkey’s support for Azerbaijan against Armenia, a Russian ally, in recent fighting in the southern Caucasus. Turkey has proclaimed full support for Azerbaijan, drawing accusations from Russia that it is facilitating the deployment of Syrian fighters to the Caucasus.