At least 11 Islamist fighters and five civilians were killed as a ballistic missile struck a building in northwest Syria during a meeting between rebel groups, a monitor said Monday.

"Eleven fighters from (Al-Qaeda affiliate) Al-Nusra Front and other Islamist groups were killed on Sunday, along with five civilians, when a ballistic missile hit a police station being used as a court in Salqin" in Idlib province, the Syrian Observatory for Human Rights said.

The Britain-based monitoring group said it was unclear if the missile was fired by Russian or Syrian forces.

The monitor said the toll could rise further because a number of people had been seriously wounded in the strike.

The missile struck during a reconciliation meeting between members of Al-Nusra and the conservative Islamist militant group Ahrar al-Sham, who had exchanged fire in the town earlier the same day.

The two groups are leading members of the Army of Conquest alliance, a powerful coalition that took control of Idlib province last year.

Staunch regime ally Russia began an aerial campaign in Syria last September after a string of government losses, including in Idlib, and has since helped reverse some opposition momentum.

Moscow says its strikes target the Islamic State (IS) jihadist group and other "terrorists," but activists and opposition forces accuse it of targeting other rebels and often killing civilians.

Earlier in January, Russian raids in Idlib province, which borders Turkey, killed at least 81 people, including 23 Al-Nusra fighters but also 52 civilians and prisoners.

According to the Observatory, more than 1,000 civilians have been killed in Russian strikes since they were launched on September 30.

The raids have also killed nearly 900 IS fighters, and more than 1,100 militants from other opposition groups, including Al-Nusra.

Moscow has dismissed claims of civilians deaths in its operations as "absurd".

Overall, more than 260,000 people have been killed in Syria's war which began with anti-government protests in March 2011.