BEIRUT — A government airstrike on a bakery in a rebel-held town in Syria killed more than 60 people on Sunday, activists said, casting a pall over a visit by the international envoy charged with negotiating an end to the country’s civil war.

The strike on the town of Halfaya left bodies and debris up and down a street, and more than a dozen dead and wounded people were trapped in tangled heap of dirt and rubble.