Updated

Government troops battling al-Qaida fighters in southern Yemen have made inroads into the militants' strongholds, but the offensive on a strategic city has slowed because of concerns the extremists could launch a surprise counterattack, military officials said Thursday.

Backed by heavy artillery and warplanes, Yemeni troops have advanced into Zinjibar, the capital of Abyan province, which fell under the control of al-Qaida-linked fighters last year as the country was engulfed by political turmoil that led to the ouster of longtime leader Ali Abdullah Saleh.

If the military were to capture Zinjibar, it would deal a heavy blow to al-Qaida by depriving it of a key base and scattering its fighters to smaller towns and mountain areas of the south.

Yemeni officials said the assault on Zinjibar, which is part of the government's broader offensive aimed at uprooting al-Qaida in the south, has slowed down in part because of poor intelligence in the city. They said army commanders were unsure whether most of the militants there have been killed, simply fled the battle or retreated for tactical reasons and were preparing a counteroffensive.

The U.S. is deeply concerned about al-Qaida's branch in Yemen, known as al-Qaida in the Arabian Peninsula, which was behind three failed bomb plots on U.S. soil. Washington has thrown its support behind Saleh's successor, Abed Rabbo Mansour Hadi, who has vowed to tackle the threat from al-Qaida in the country.

As part of the fight, Hadi is restructuring the military and removing Saleh's loyalists from key posts, though there are concerns that the former leader, who stepped down in February, is still obstructing reforms and trying to retain influence through his cronies.

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.

Yemeni officials point to an ongoing power struggle between Hadi and Saleh.

In one recent example, officials say Saleh's son Ahmed, who commands the elite Republican Guard, has sought to undermine a Hadi-appointed commander of one of the Guard's battalions. The officials say Ahmed has been stirring up dissent within the battalion, making it impossible for the Hadi appointee to assume control.

Three foreign diplomats told The Associated Press that they have met with Ahmed to try to convince him to allow the new commander to take his post.

One diplomat warned Ahmed this week that by blocking the commander's appointment, he is undermining the power-transition deal that his father signed and that assured him immunity from prosecution.

Washington is also directly aiding the Yemeni military in its latest offensive. U.S. troops 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 AQAP.

In a nationally televised speech late Thursday, Yemeni Defense Minister Mohammed Nasser Ahmed claimed that the town of Lawder near Zinjibar had been cleared of militants and fully secured. He also thanked his troops and local tribesmen who fought along side the armed forces for their help.

In Lawder, residents celebrated the end of the military operations there. Men and women took to the streets, and some residents fired in the air in celebration.

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

Also Thursday, Yemeni military officials said a suspected U.S. drone-fired missile struck two vehicles in the city of Shibam in 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.

Yemeni warplanes, meanwhile, struck a vehicle in the town of Shaqra, killing six suspected militants, officials said.

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