SANAA, Yemen — Yemeni troops battling al-Qaida fighters in the country's south have forced them to retreat but military officials said Thursday the push in a major southern city is going slowly because of concerns the militants could stage a surprise counterattack.

The military, backed by heavy artillery and warplanes, is advancing inside the city of Zinjibar, the provincial capital of Abyan, the officials said. The city fell into al-Qaida's hands during last year's popular uprising against the now ousted leader Ali Abdullah Saleh.

Zinjibar's capture would deal a heavy blow to al-Qaida as it would leave the terror group's fighters scattered in smaller towns and mountain areas of the south.

The push in Zinjibar is part of Yemen's offensive along several fronts to uproot al-Qaida from its southern strongholds at a time when the U.S. has thrown its support behind the new Yemeni president and Saleh's successor, Abed Rabbo Mansour Hadi.

Hadi has vowed to combat al-Qaida by restructuring the military and replacing Saleh's loyalists from key posts, though there are concerns that Saleh, who stepped down in February, is still obstructing reforms and trying to retain influence through his cronies.

The latest offensive began Tuesday, with the military using warplanes and heavy artillery to clear the way for an assault by ground troops on towns where al-Qaida fighters are either operating or in control.

Yemen's army is receiving direct help from U.S. troops, who are operating from a desert air base near the main battle zones to help coordinate assaults and airstrikes, according to Yemeni officials.

The officials said it was the most direct American involvement yet in the country's expanding campaign against al-Qaida's branch in Yemen, which has been blamed for directing a string of unsuccessful bomb plots on U.S. soil from its hideouts in the impoverished country at the tip of the Arabian Peninsula.

On Wednesday, President Barack Obama signed an executive order allowing the Treasury Department to freeze the U.S.-based assets of individuals who the White House says "threaten the peace, security and stability" of Yemen. The order was meant as a deterrent against future action and does not immediately levy any penalties against specific people or organizations.

On Thursday, Yemeni officials said the military intelligence in Zinjibar was poor and that army commanders were unsure whether most of the al-Qaida fighters there have been killed, simply fled the battle or retreated for tactical reasons and were preparing a counteroffensive.

To the east of Zinjibar, Yemeni military officials said a suspected U.S. drone-fired missile struck two moving vehicles in the city of Shibam in the southern Hadramawt province. The officials said the strike destroyed one of the vehicles, killed its three occupants who were believed to be linked to al-Qaida, and wounded two in the second car.

Also in the south, bodies of 11 Yemeni army soldiers and civilian volunteers fighting alongside the military were found after al-Qaida retreated from the area near the city of Lawder, officials said. The victims were believed to have been killed execution-style, they said.

"Most of the areas surrounding Lawder are clear of al-Qaida now," one military official said.

All the officials spoke on condition of anonymity because they were not authorized to talk to the media.