Dozens of fighters from a Turkish-backed rebel group have been killed in Russian airstrikes in Northwestern Syria, according to a British war monitoring group.

At least 56 rebel fighters were killed and 100 more injured in airstrikes by Russian warplanes on a military training camp in Northwestern Syria on Monday, according to the Britain-based Syrian Observatory for Human Rights.

The camp is located in Idlib province, close to the Turkish border and belonged to Faylaq al-Sham, an alliance of Sunni Islamist rebels aligned with Turkey, the war monitor said.

Russia, an ally of the Syrian government, brokered a fragile truce alongside Turkey in March aimed at curbing the Damascus-led military offensive against the country's last remaining rebel stronghold in Idlib. The fighting has displaced nearly a million people.

The March truce has largely held, apart from intermittent airstrikes and bombardment in the area. A US drone strike on Thursday killed five civilians and 17 jihadi fighters according to the Observatory.

A decade of war

Syrian government forces, backed by Russian air power, have driven rebels in a pocket of territory near the border with Turkey.

Ankara sent thousands of Turkish troops into the region this year in an effort to halt the Syrian military and stem the flow of refugees into Turkey.

Syria has seen more than nine years of conflict between government and rebel fighters since the 2011 protests against President Bashar Assad.