By Randall Mikkelsen

WASHINGTON, Jan 31 (Reuters) - A senior Al-Qaeda leader in Afghanistan, described by Western authorities as one of Osama bin Laden's top six lieutenants, has been killed, U.S. officials and an al-Qaeda-linked Web site said on Thursday.

The Web site said Abu Laith al-Libi 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.

The prominence of the Libyan-born militant in al Qaeda was shown 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 2001.

A statement on the Al-Fajr Media Center, a Web site often used by Islamic militants, said Libi had been "martyred along with a group of his brothers on the territory of Islamic Pakistan."

"May God accept (Libi as a martyr)," the Ekhlaas.org Web site said. A Western official and senior U.S. defense official confirmed the Web site's report.

There was no official word in Washington on the circumstances of Libi's death, which coincided with intensive contacts between U.S. and Pakistani security officials after a year in which violence escalated sharply in Afghanistan.

A Pakistani daily, The News, said 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 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.

"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," the Western official said.

TOP AL QAEDA LEADER

He said Libi was one of the top six leaders in al Qaeda's global structure and said it was reasonable to suppose his reach extended into Pakistan. Al Qaeda has reconstituted its leadership in Pakistan's North Waziristan border area.

An intelligence official told Reuters in Pakistan the attack was believed to have been carried out by a pilotless U.S. Predator aircraft flown across the nearby border with Afghanistan.

The Pakistani authorities have not confirmed the attack, and the Pentagon has denied taking part. But the Defense Department does not speak for the Central Intelligence Agency, which operates Predators. The CIA declined to comment.

Washington, alarmed by growing al Qaeda activity including a number of plots uncovered in western countries with links to Pakistan militants, has renewed a push for greater Pakistani cooperation against al Qaeda. Senior U.S. security officials including CIA Director Michael Hayden were in the area this month.

"There may be an opportunity here (Pakistan) to better focus how we can help in that area, both for our own interests and for their interests," U.S. military Joint Chiefs of Staff Vice Chairman Gen. James Cartwright said earlier this month.

While some analysts said it was a significant blow, Seth Jones of the Rand Corp think tank said the killing would not end the threat of al Qaeda in the border area. "This is just one strike in an area that's infiltrated by militants."

The violence in Afghanistan in 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.

Libi may have been linked to a suicide bomb attack at the Bagram military base during a visit by U.S. Vice President Dick Cheney last February, said Security analyst David Heyman of the Center for Strategic and International Studies.

The Western official, who asked not to be identified, said it was "certainly plausible to think that at a minimum he had knowledge of it if not active involvement."

Heyman said Libi had called last year for the assassination of Pakistani President Pervez Musharraf, which may have made Pakistan more open to easing its reluctance to accept U.S. action in North Waziristan and allow a Predator attack.

Analysts said Libi's militancy extended back to the 1980s, with ties to Algeria and Libya.

He was head of the Libyan Islamic Fighting Group, which had long been active in Afghanistan and formally joined al Qaeda last year in a demonstration of the group's ability to expand. (Additional reporting by Firouz Sedarat in Dubai; Editing by David Storey)

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