"Twenty civilians were killed and dozens wounded" in the attack, a medical official said. Witnesses said the provincial capital's market was targeted.
The attack came hours after a Saudi-led coalition air strike killed seven rebels in Marib, where Apache helicopters have also joined the fight against Huthi Shiite insurgents and their allies.
The coalition strike targeted a military vehicle used by the rebels in Marib province, while other air raids targeted rebel positions in neighbouring Shabwa, military sources said.