Residents in the coastal city of Zinjibar told news agencies that they had seen warplanes dropping bombs in an effort to dislodge the militants and that the army had begun artillery shelling.

The fall of Zinjibar to self-styled holy warriors who claimed to have “liberated” it from “the agents of the Americans” fed into Western fears that militants sympathetic to Al Qaeda could exploit the breakdown of authority to take control of territory.

Political opponents of Yemen’s embattled president, Ali Abdullah Saleh, portrayed the takeover as a ploy by Mr. Saleh to prove to wavering allies why they needed to keep him in power. While Mr. Saleh, who has faced months of massive street protests demanding his ouster, has frequently warned that militants would take over the country if he left, there was no evidence on Sunday that he had any role in allowing Zinjibar to fall.