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By Firouz Sedarat

DUBAI, Jan 31 (Reuters) - One of Osama bin Laden's top lieutenants who commanded militant forces in Afghanistan has been killed, U.S. officials and an al Qaeda-linked Web site said on Thursday.

The Web site said he had been killed in Pakistan, suggesting he may have died in a suspected U.S. missile strike that killed up to 13 foreign militants in Pakistan's North Waziristan border area this week.

Abu Laith al-Libi's prominence in al Qaeda was highlighted last year by his appearance in a video with the group's deputy leader. He was the first spokesman to announce bin Laden had survived the U.S.-led invasion of Afghanistan in late 2001.

U.S. media have said the Libyan-born militant was believed to be behind a suicide bombing in February 2007 that killed 23 people outside the main U.S. Bagram base in Afghanistan during a visit by U.S. Vice President Dick Cheney.

Libi was "martyred along with a group of his brothers on the territory of Islamic Pakistan", said a statement from the al Qaeda-linked Al-Fajr Media Centre posted on a Web site often used by Islamic militants.

"May God accept (Libi as a martyr)," the Ekhlaas.org Web site said.

Asked whether Libi had been killed, a senior U.S. defense official in Washington said: "Yes."

A Western official, who declined to be named, said: "This individual is in the top half dozen figures in al Qaeda ... who has a long record of military activity on behalf of al Qaeda.

The official said: "You're talking about a very seasoned commander ... the top or one of the absolute top military commanders that al Qaeda had in the region. That's what they lost."

KILLINGS

Some Western anti-terrorism analysts said past killings of leading al Qaeda figures had shown there were usually others ready to fill the gap in the organisation's ranks.

"This is just one strike in an area that's infiltrated by militants," said Seth Jones of the Rand Corp think-tank.

But David Heyman of the Center for Strategic and International Studies said: "This is a very significant success."

The violence in Afghanistan over the past two years has been the bloodiest since the Islamist Taliban were ousted in 2001. More than 10,000 people, including over 300 foreign troops, have been killed, according to estimates by aid groups.

A Pakistani daily, The News, reported the suspected U.S. strike on Monday had targeted Libi and another senior figure, Obaidah al Masri, though residents in the tribal area had said the attack had targeted second or third tier al Qaeda leaders.

Tribesmen in the area had said a deputy of Libi had been staying in the area, which borders Afghanistan, and was among the dead, according to an intelligence official.

Libi appeared in a video issued in November with al Qaeda's second-in-command Ayman al-Zawahri to announce that a Libyan Islamist group had joined the organisation.

Islamist Web sites have carried messages from Libi, including one in May in which he said al Qaeda in Afghanistan was willing to exchange prisoners with Britain and other Western countries.

In October the U.S. military in Afghanistan named Libi among several "mid-level" al Qaeda and Taliban leaders and offered a $200,000 bounty for him, U.S. media reported. In 2002, Libi was the first from the group to bring news that bin Laden and Taliban leader Mullah Omar were alive after the U.S.-led invasion of Afghanistan. (Additional reporting by Kristin Roberts in Washington)

(Editing by Ralph Gowling)

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