WASHINGTON - A US military offensive in southern Afghanistan could push Taleban fighters into Pakistan, whose troops are already struggling to combat militants, the chairman of the US Joint Chiefs of Staff said on Thursday.

The comments by Admiral Mike Mullen raised the prospect that Pakistan, waging battles with militants who have forced about 2 million people from their homes, could face even greater turmoil in the months ahead.

The US is pouring thousands of troops into Afghanistan this year to try to reverse gains by a resurgent Taleban, particularly in its southern heartland.

Mr Mullen said the US had a clear national security interest in taking on the Taleban.

'They want Afghanistan back. We can't let them or their Al-Qaeda cohorts have it. We can't permit the return of the ...very same safe havens from which the attacks on 9/11 were planned and resourced,' Adm Mullen said.

Senator Russ Feingold expressed concern about a possible spillover: 'We may end up further destabilizing Pakistan without providing substantial lasting improvements in Afghanistan.'

'Weak civilian governments, an increased number of militants and an expanded US troop presence could be a recipe for disaster for those nations in the region as well as our own nation's security.' He suggested the presence of US forces in Afghanistan after the Sept 11, 2001, attacks on the United States also pushed militants into Pakistan, where Al-Qaeda regrouped.

Adm Mullen said he thought a spillover could be avoided because both Pakistani and US forces were aware of the possibility and were planning measures to prevent it.

Adm Mullen also said the opium trade, which helps fund the Taleban, must be eliminated in Afghanistan and he acknowledged Washington and its allies had failed to tackle the problem.

He said the US and other nations had to step up efforts to provide alternative livelihoods for farmers who grow opium poppies. -- REUTERS