By Sayed Salahuddin

KABUL, Feb 10 (Reuters) - Afghanistan said on Sunday it backed a senior U.S. official's assertion that al Qaeda leader Osama bin Laden and Taliban chief Mullah Mohammad Omar were operating from neighbouring Pakistan.

Afghanistan has long said al Qaeda and Taliban leaders receive safe refuge in parts of Pakistan's lawless tribal regions, souring relations between the neighbours. But since a large tribal council last August, the two countries agreed to work more closely to fight the joint militant threat and ties have improved.

The U.S. official said Bin Laden, his deputy Ayman al-Zawahri and other network members were operating out of Pakistan's Federally Administered Tribal Areas, bordering Afghanistan.

Mullah Omar and other ousted Afghan Taliban leaders, meanwhile, were directing insurgent operations in Afghanistan from the Pakistani city of Quetta, said the U.S. official, who spoke on condition of anonymity.

Pakistan has rejected the charge, but a spokesman for Afghan President Hamid Karzai welcomed it.

"We are glad that finally a high-ranking American official confirmed this matter," said spokesman Humayun Hamidzada.

"The government of Afghanistan has said for years the administration centres, havens and regrouping bases of the enemies of Afghanistan and Taliban are outside Afghanistan."

INSURGENCY RELAUNCHED

U.S.-led and Afghan troops overthrew the Taliban government in 2001 after its leaders refused to hand over bin Laden in the wake of the Sept. 11 attacks on the United States.

Many militant leaders are believed to have fled to Pakistan, but Taliban rebels regrouped and relaunched their insurgency two years ago with a wave of guerrilla attacks and suicide bombs. More than 6,000 were killed in Afghanistan last year alone.

Hamidzada said the problem had to be dealt with at source.

"The government of Afghanistan in the past has repeatedly said the roots of terrorism, its original sources and bases should be dealt with," he said.

"Certainly, the war in Afghanistan should continue, but the war should be taken to the source of terrorism where it is. We are not naming any country."

The assertion by the U.S. official about the presence of militants' leaders in Pakistan comes after the killing in January of top al Qaeda commander Abul Laith al-Libi in a suspected U.S. missile attack in a Pakistani tribal area bordering Afghanistan.

It also coincides with an increase of attacks, including suicide bombings, by militants in Pakistan.

A Taliban spokesman said Mullah Omar was leading the insurgency from within Afghanistan and said the U.S. official was preparing the ground for a military operation in Pakistan.

"This is false. Mullah Omar is not in Quetta but present in Afghanistan and commanding the Taliban," Qari Muhammad Yousuf told the Pakistan-based Afghan Islamic Press.

"The claims that Mullah Omar is in Quetta or any other place are aimed at finding a pretext for conducting an operation in the concerned area." (Editing by Alex Richardson)

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