A US air strike in northern Syria did not hit a mosque, rather a nearby building with "dozens" of Al-Qaeda members inside, the Pentagon said Friday, as it showed reporters a photo of the strike's aftermath.

"The mosque is still standing and relatively unscathed," Pentagon spokesman Navy Captain Jeff Davis said, promising the image would be made available to the public.

The Britain-based Syrian Observatory for Human Rights earlier said warplanes had targeted a mosque in Aleppo province during evening prayers, killing 42 people.