Warplanes struck a mosque in the rebel-held village of al Jina, in north-west Syria, killing at least 42 people and wounding dozens, the Syrian Observatory for Human Rights, a Britain-based war monitor said on Thursday.
The jets struck the village, south-west of Atarib near Aleppo, as the mosque was full of worshippers at evening prayer, said the Observatory, which monitors the Syrian war via a network of contacts across the country.
The US military said it carried out an air strike on Thursday against al Qaeda militants, killing several, at a meeting location in Idlib province, but it was unclear if it was referring to the attack on al Jina.
The statement from US Central Command made no mention of civilian casualties.
Syrian and Russian military have also carried out many air strikes in Idlib and Aleppo provinces during the war.
The area is located in one of the main rebel-held parts of Syria, the north-west that includes Idlib province and the western parts of Aleppo province, and its population has been swollen by refugees, UN agencies have said.
Rebels in north-west Syria fighting to oust President Bashar al Assad also include groups supported by Turkey, the United States and Gulf monarchies.