By Mohammed Mukhashaf

ADEN (Reuters) - An apparent U.S. drone attack killed at least six suspected al Qaeda-linked militants in southern Yemen on Saturday, while Yemeni government forces killed 15 others in a new offensive against insurgents, local and military officials said.

U.S. officials said earlier this week they had thwarted a plot by the Yemen-based al Qaeda in the Arabian Peninsula (AQAP) to arm a suicide bomber with a non-metallic device, an upgraded version of the "underwear bomb" carried onto an airliner on Christmas Day 2009.

AQAP, a regional branch of the militant network, has plotted overseas attacks that have been prevented but raised major concern for Washington, which is trying to wipe out suspected AQAP operatives with drone and missile strikes.

An air strike believed to have been carried out by a U.S. drone set two vehicles on fire, killing seven suspected insurgents in the southeastern province of Shabwa, a local official told Reuters. One of the dead was an Egyptian, he said.

The Defense Ministry said in a text message to reporters that six al Qaeda militants were killed and that three vehicles had been targeted by the "air strike", without elaborating. Yemen and Washington do not acknowledge U.S. drone attacks.

It was the latest in a series of reported drone attacks on militants in the south of the impoverished Arab country who exploited mass protests last year against then-President Ali Abdullah Saleh to seize large swathes of territory, including Zinjibar, the capital of restive Abyan province.

Last week, the U.S. Defense Department said Washington had resumed training of Yemeni armed forces to bolster the fight against al Qaeda, after a suspension during the political upheaval that ousted Saleh.

RESIDENTS TOLD LEAVE BATTLE ZONES

Residents said Yemeni air force planes dropped leaflets on Saturday urging civilians to leave areas held by militants targeted by the army offensive, prompting a mass exodus from parts of Abyan.

Fifteen insurgents as well as five soldiers and an army officer were killed in fighting on Saturday, a military official who did not want to be further identified told Reuters.

"A force of about 20,000 men is taking part in this offensive, ordered by President Abd-Rabbu Mansour Hadi to free the cities of Zinjibar and Jaar," he said. Navy units would also be used in operations along Abyan's coast on the Gulf of Aden.

Yemen's fractured state and dysfunctional security apparatus have provided al Qaeda's regional wing with a suitable breeding ground for bomb plots on U.S. and other Western targets.

But tribal leaders in parts of Yemen where drone attacks aimed at AQAP have killed civilians say the air strikes are turning more and more people against the government and the United States.

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.

In March, the United Nations refugee agency UNHCR warned that Yemen was facing a new wave of internal displacement as tens of thousands of civilians fled tribal clashes in the north and fighting with militants in the south, in a country with about half a million displaced people.

Hadi, who had been Saleh's vice-president, was elected unopposed in February under a U.S.-backed power transition plan brokered by Yemen's Gulf neighbors to end the political turmoil.

Hadi, who has vowed to defeat al Qaeda and unify the army, also faces challenges from Shi'ite Muslim rebels in the north and tribal secessionists in the south.

(Additional reporting by Mohammed Ghobari in Sanaa; Writing by Firouz Sedarat; Editing by Mark Heinrich and Janet Lawrence)