Russian air strikes have killed 45 members of a rebel group in the northwestern Syrian province of Idlib, the Syrian Observatory for Human Rights monitor said Sunday.
It was unclear why the strikes, which took place Saturday, targeted members of the Faylaq al-Sham rebel group, which has taken part in peace talks supervised by Moscow in the Kazakh capital Astana.
The strikes hit the group in the province of Idlib, which is part of a so-called "de-confliction" zone agreed in a deal between Syrian regime allies Russia and Iran, and rebel backer Turkey.
The Observatory initially reported a lower toll, but said the figure had risen as bodies were recovered after the strike on one of the rebel group's headquarters on the outskirts of the village of Tal Mardikh.
Faylaq al-Sham is an Islamist rebel group considered close to the Muslim Brotherhood movement.
It has fought against the former Al-Qaeda affiliate that now effectively controls nearly all of Idlib after chasing its former rebel allies from their positions in fighting this summer.
A spokesman for Faylaq al-Sham confirmed the group's headquarters had been targeted, despite their participation in the last round of talks in Astana.
"Our participation in Astana does not in any way mean that Russia is a friendly or neutral country," Idriss al-Raed told AFP.
"The Russian bombing is not surprising, since its policy since its intervention in Syria is based on criminality and killing," he added.
Russia began an intervention in Syria in support of ally President Bashar al-Assad in 2015, and has helped the regime win back large parts of the country.
Moscow also helped broker a deal for four truce zones, one of which includes Idlib province, in an agreement with Iran and Turkey inked in May.
The deal excludes jihadists, but is intended to otherwise halt fighting in the agreed areas.
The three other zones are around the capital Damascus, in southern Syria and in the centre of the country.
More than 330,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.