The fall of the coastal city 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.
Yemen's air force bombed the positions of al Qaeda and Islamist militants who have taken control of the southern coastal city of Zinjibar, residents said on Monday.
They also said the army is shelling the city with artillery.
Several hundred suspected al Qaeda and Islamist militants took over the Gulf of Aden city a few days ago and have been battling locals and government soldiers who are trying to regain control.
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 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.
The United States has until recently backed Mr. Saleh as an ally in the fight against al Qaeda, whose Yemeni branch is considered one of the most active terrorist threats against the United States and Europe.
The militants who took over the town of Jaar in March and Zinjibar this weekend are not confirmed as al Qaeda members, but the volatile province of Abyan, where both cities are located, is filled with citizens who are sympathetic to the group.
Residents said that despite the efforts of a handful of soldiers, who mounted a brief defense, the town fell quickly and easily to several hundred militants.