FBI director Robert Mueller visited Yemen on Tuesday, pledging to help quell an Islamist insurgency there, as security and government sources said a U.S. drone had killed a prominent al-Qaida leader linked to an attack on a French oil tanker.

In a meeting with President Abd-Rabbu Mansour Hadi, who took office earlier this year, Mueller promised that the United States would continue to support Yemen "with full force" in all respects.

"Mueller visits Yemen on an annual basis so this is not a special or secret occasion," said Mohammed Al-Basha, Yemen's embassy spokesman in Washington. "President Hadi emphasized that he is strongly committed to combating extremism and working with the U.S. to counter the mutual threat of terrorism."

Separately, the Yemeni embassy in Washington said on Tuesday that Mohammed Saeed al-Umda, convicted in 2005 of involvement in the 2003 attack on the Limburg oil tanker, had been killed in an air strike on his convoy in the oil-producing province of Maarib on Sunday. It did not specify whether it was a U.S. strike.

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Umda, described by the embassy as Yemen's fourth-most wanted man, had received military training under Osama bin Laden in Afghanistan and was in charge of the group's finances, a security source said.

The United States has repeatedly used drones to target suspected al-Qaida militants, who have been emboldened by a year of political upheaval in the impoverished state.

Exploiting mass protests against former president Ali Abdullah Saleh's 33 years in office, militants linked to al-Qaida last year seized large swathes of territory in southern Yemen, including at least two towns.

Yemen's army, which split into two factions during the uprising that eventually unseated Saleh, has been battling to get the upper hand against the militants.

On Tuesday, the Defense Ministry put the total number of militants killed in the volatile southern Abyan province over the past two days in the latest bout of fighting at 52.

It said the Yemeni army has also seized some government offices from militants as they pushed deep inside the provincial capital of Zinjibar.

In an emailed statement, Ansar al-Sharia, an al-Qaida-affiliated group, said its fighters has blocked the army advance, and challenged the authorities to issue "just one recent photograph showing troops inside the city (Zinjibar)".

The statement did not refer to the drone attack.

Saleh's half-brother steps aside
President Hadi is trying to reform the army, but has run up against the vested interests of Saleh's relatives and allies still in charge of the military and security establishment.

In a modest victory for Hadi, Mohammed Saleh al-Ah mar, a half-brother of Saleh left his post as air force commander on Tuesday. Earlier this month, he shut down the capital's airport and grounded all flights to protest against his removal in a direct challenge to Hair's authority.

"The handover has taken place as stated in the decree issued by the president," U.N. envoy Jamalco Benchmark told reporters in Sanaa. "It was a smooth handover with no conditions whatsoever."

The development is the first time Hadi has succeeded in distancing Saleh's relatives from power - but Saleh's son, nephew and other allies remain in place as heads of key military units.

Benchmark, who helped push through the plan under which Saleh eventually left office after more than a year of popular unrest, persuaded the former president to lean on his half-brother to step aside, a government official said.

"The U.N. envoy Jamalco Benchmark conducted negotiations to convince the former president of the need to implement the decree to remove his half brother from the leadership of the air force," said the official, on condition of anonymity.

General Rash Ali Nasser al-Jund replaces Saleh's half-brother as head of the air force. Mohammed Saleh al-Ah mar has been appointed an assistant to the defence minister.

Hadi, who had been Saleh's vice-president, was elected president unopposed in February under a U.S.-backed transition plan broke red by Yemen's wealthy Gulf neighbors, anxious to halt a slide into mayhem.

In a separate incident on Tuesday, a local security source said the head of political security in southern Lahej province had survived an assassination attempt that had left him with severe injuries. The source said a bomb had been attached to his car and it exploded when he started the engine.

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Updated at 11:17 a.m. ET: Yemeni security and military officials revised the number of suspected militants killed in a U.S. airstrike Tuesday down to four people killed, and said one of the victims was a man suspected of involvement in the 2000 bombing of the USS Cole.

Tribal officials in the southern Abyan province said the strike hit the militants late Monday as they were holding an important meeting at the school. Yemeni security officials had originally put the death toll at 15 people but later lowered that figure. They also said 12 militants were wounded in the strikes.

They said one of the suspected militants killed was named Abdel-Monem al-Fathani who was involved in the bombing of the USS Cole, which killed 17 American sailors and injured 39 others. The attack on the U.S. destroyer occurred while it was in the Yemeni port of Aden for refueling.

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A Western official in Washington confirmed the U.S. carried out a strike against suspected leaders from al-Qaida in the Arabian Peninsula, saying initial indications were that five people were killed. The official did not say where the strike occurred or specify whether it was carried out by a drone or a warplane.

Updated at 9:15 a.m. ET: U.S. airstrikes targeting leaders from Yemen's active al-Qaida branch killed 15 suspected militants, Yemeni officials said on Tuesday.

Yemeni security and military officials said missiles struck a school and a car in Abyan province in an area between Lauder and Mood where the militants were believed to be hiding.

NBC News also reported Tuesday that members of a Yemeni tribe had kidnapped six United Nations aid workers. They were demanding money and the release of one of their members from jail.

According to NBC, the U.N. aid workers include a German, French, Colombian, Iraqi and Lebanese.

A Western official in Washington confirmed the U.S. carried out a strike against suspected leaders from al-Qaida in the Arabian Peninsula, but said initial indications were that five people were killed. The official did not say where the strike occurred or specify whether it was carried out by a drone or a warplane.

All the officials spoke on condition of anonymity because they weren't authorized to release the information.

Also Tuesday, Yemen's information minister, Ali al-Amrani, escaped an assassination attempt in the capital Sanaa, an aide said.

Unidentified assailants opened fire on the minister's car after the weekly cabinet meeting and then fled, Amrani's secretary, Abdel-Basset al-Qaedi, said.

The minister was not hurt, he said.

Updated at 2:33 a.m. ET: At least 11 people, including several alleged local al-Qaida leaders, were killed in an overnight airstrike in southern Yemen, local residents said Tuesday.

They said an unidentified drone attacked two vehicles east of the city of Lauder in Abyan province in southern Yemen.

A tribal leader said at least four of those killed were local al-Qaida leaders. Residents said no civilians were hurt in the airstrike.

The United States has used drones repeatedly to attack al-Qaida militants in Yemen. Last September, a U.S. drone killed U.S.-born cleric Anwar al-Awlaki, described by U.S. officials as "chief of external operations" for al-Qaida in Yemen.

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Al-Qaida has exploited unrest and protests against President Ali Abdullah Saleh to strengthen its hold on remote areas in southern Yemen in recent months.

An opposition-led government has been set up in Yemen after Saleh agreed in November to transfer authority to his deputy ahead of presidential elections in February.

• Yemeni president arrives in US for medical treatment

But protests have continued and activists are pressing on with demands that Saleh be tried for alleged killings of demonstrators and that the government is purged of members of his family.

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Reuters and The Associated Press contributed to this report.