The Syrian Observatory for Human Rights reported that Russian warplanes bombed the rugged Jabal al-Akrad region of Latakia province on Monday afternoon.
The commander of a Western-backed Free Syrian Army brigade and the families of rebels were among the dead, it added.
Russia said it had targeted Islamic State militants in Latakia on Monday.
However, activists say the jihadist group has little or no presence in the coastal province, which is a stronghold of President Bashar al-Assad’s Alawite sect.
Moderate and Islamist rebel groups took control of Jabal al-Akrad in 2012.
From positions in the mountainous region, they have been able to shell the city of Latakia’s airport and Qardaha, the president’s ancestral village.
Jabal al-Akrad has been targeted several times by Russian warplanes since Moscow launched an air campaign to bolster Mr Assad on 30 September.
The UK-based Syrian Observatory’s director, Rami Abdul Rahman, said dozens of people were also wounded in Monday’s raids, and that the death toll was likely to rise because many were in a serious condition.
The Local Co-ordination Committees (LCC), an opposition activist network, reported that 57 people were killed in Latakia province on Monday, most of them in Russian air strikes.
It said the attacks were focused on the village of Bisharfeh, and that the casualties included many children and women.
It also identified the rebel commander who died as Basil Zimmo, the chief-of-staff of the FSA’s 1st Coastal Division and a former captain in the Syrian military.
The 1st Coastal Division is believed to have been supplied with US-made TOW anti-tank missiles as part of US-backed programme assisting moderate rebels.
The commander of another such group, the Nour al-Din al-Zinki Brigades, was killed in fighting south of the city of Aleppo on Monday, as a government offensive in the area has reportedly displaced tens of thousands of people.