US-led forces in Afghanistan will move into Pakistani territory to destroy Taliban and other extremist groups if Islamabad cannot do the job by itself, the US envoy in Afghanistan warned on Monday.

"We cannot allow this problem to fester indefinitely," said Zalmay Khalilzad, also the special presidential envoy in Kabul, who only in March angered Pakistan by alleging that Taliban and Al Qaeda fugitives were launching attacks into Afghanistan from Pakistani soil.

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Although Pakistan's recent army deployment into tribal areas to destroy Taliban sanctuaries along the Afghan border was 'positive and hopeful', he said the Taliban and other extremist groups continued to be able 'to base, train and operate from that country's territory'.

The radical Taliban Islamic militia was ousted in 2001 for offering sanctuary to Al Qaeda leader Osama bin Laden.

Khalilzad told a forum organised by the Centre for Strategic and International Studies in Washington that unless Pakistan rooted out Taliban sanctuaries, it would be difficult to fully eliminate security problems in the south and east of Afghanistan.

"We have told the Pakistani leadership that either they must solve this problem or we will have to do it for ourselves," he said. "We prefer that Pakistan takes responsibility, and the Pakistani government agrees."

Khalilzad said the US-led coalition was prepared to help Pakistan President Gen Pervez Musharraf.

"However, one way or the other, this problem will have to be dealt with," he said.

Pakistani authorities in March stepped up efforts to get rid of Al Qaeda fugitives from a semi-autonomous tribal belt bordering Afghanistan. But it re-channelled its efforts into negotiations with the region's tribes after a largely disastrous operation against some 500 fighters holed up in the mud-walled fortress homes of rebel tribal elders.

The 12-day battle by 7,500 Pakistani troops resulted in the capture and death of some 200 fighters and local tribal supporters, and the death of at least 46 troops.

No significant Al Qaeda figures were among those killed or captured, and the remainder escaped through underground tunnels or by slipping across a troop cordon.

Khalilzad also said that it would take a sustained commitment of at least five years by the United States and its partners to consolidate the victory over extremism and terrorism in Afghanistan.

"The next major task is for the United States - both parties and both branches of the government - to make that commitment and to put in place a five-year program that will enable Afghanistan to stand on its own feet," he said. "This is important and worthwhile as we face budget deficits and substantial costs in Iraq."

Khalilzad did not elaborate on the program.

The US had pledged an additional $1billion in aid to Afghanistan for 2004 on top of the $1.2billion it had announced earlier.

Khalilzad said the immediate challenge was the holding of presidential and parliamentary elections in Afghanistan in the coming months. The challenge is not security but rather logistical and operational, including registering nine to 10 million Afghans as voters.

So far the United Nations, which had led responsibility for the elections, had registered 1.5 million people.