* FBI chief vows to help crush insurgency
* President Hadi says committed to working with U.S.
* One of Yemen's top al Qaeda men killed in air strike
* Saleh's half-brother leaves air force command (Adds comment from U.S. officials, paragraphs 6-7)
SANAA, April 24 (Reuters) - FBI director Robert Mueller visited Yemen on Tuesday, pledging to help quell an Islamist insurgency, as security and government sources said a drone had killed a prominent al Qaeda 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 the United States would 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 emphasised that he is strongly committed to combatting extremism and working with the U.S. to counter the mutual threat of terrorism."
Yemen's embassy in Washington said on Tuesday that Mohammed Saeed al-Umda, convicted in 2005 of involvement in the 2002 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.
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.
In Washington, U.S. officials indicated that there had been other similar air attacks recently against militant targets in Yemen. U.S. cooperation with Yemeni authorities on counter-terrorism issues appears to have improved somewhat since Hadi took over from long-time president Ali Abdullah Saleh earlier this year.
The officials, who spoke on condition of anonymity, said they were aware of reports of al-Umda's death in such an attack but did not have final confirmation. U.S. sources indicated that the airstrike in which the Yemenis say he was killed was carried out by a missile fired from a CIA-operated drone aircraft.
The United States has repeatedly used drones to target suspected al Qaeda militants, who have been emboldened by a year of political upheaval in the impoverished state.
Exploiting mass protests against former president Saleh's 33 years in office, militants linked to al Qaeda 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 Defence Ministry put the number of militants killed in the southern Abyan province in the past two days at 52. It said the army had seized some government offices from militants as they pushed deep inside the provincial capital of Zinjibar.
In a statement, Ansar al-Sharia, an al Qaeda-affiliated group, said its fighters had blocked the army's 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.
Tribal sources in Abyan said up to 21 militants were killed in three separate clashes with pro-government tribesmen on Tuesday. Two tribal fighters died in the fighting, they said.
SALEH 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-Ahmar, 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 Hadi's authority.
"The handover has taken place as stated in the decree issued by the president," U.N. envoy Jamal Benomar told reporters in Sanaa. "It was a smooth handover with no conditions whatsoever."
It was the first time Hadi had succeeded in removing one of Saleh's relatives from power. Saleh's son, nephew and other allies remain in place as heads of military units.
Benomar, who helped push through the plan under which Saleh 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 on condition of anonymity.
General Rashed Ali Nasser al-Jund becomes head of the air force. Ahmar was appointed an assistant to the defence minister.
Hadi, who had been Saleh's vice-president, was elected unopposed in February under a U.S.-backed transition plan brokered by Yemen's wealthy Gulf neighbours, anxious to halt a slide into chaos.
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 exploded when he started the engine. (Reporting by Tom Finn and Mohammed Ghobari in Sanaa, Mohammed Mukhashaf in Aden and Mark Hosenball in Washington; Writing by Isabel Coles and Firouz Sedarat; Editing by Janet Lawrence and Eric Walsh)