US troops in Afghanistan massed close to the border yesterday for a possible attack on al-Qaeda and Taleban bases in the lawless North Waziristan tribal belt in Pakistan.

Reports from the area said that hundreds of Nato troops were airlifted across the mountains from the village of Lowara Mandi, which has been an important base for cross-border attacks in Afghanistan. Heavy artillery and armoured vehicles were also being moved into position.

The deployment followed a claim by the Afghan Government on Monday that the Pakistani Army and its spy agency had become “the world's biggest producers of terrorism and extremism”. The Pakistani Foreign Ministry accused Kabul of creating an “artificial crisis to satisfy short-term political expediencies”.

President Bush said yesterday that the US would investigate the Afghan claims to “get to the bottom of the allegation”. He said that he was troubled by the movement of extremists from Pakistan into Afghanistan.

“I certainly hope that the [Pakistani] Government understands the dangers of extremists moving in their country,” Mr Bush said.Tensions have been heightened since the deaths of nine soldiers on Sunday when insurgents overran a US base in Kunar province, close to the Pakistani border.

Western commanders say there has been a marked increase in cross-border infiltration in the past few months, fuelling the insurgency in Afghanistan. Nato troops have clashed with Pakistani units along the South Waziristan border.

US Admiral Michael Mullen, the chairman of the Joint Chiefs of Staff, made an announced visit to Islamabad at the weekend and held a series of meetings with Pakistan's top civil and military leadership.

According to well-placed sources, Admiral Mullen warned Pakistan that the US could take unilateral military action if the cross-border attacks in Afghanistan were not stopped. The US official said that some elements within Pakistani security agencies could be helping the insurgents operate from their bases in the border region.

An influential Pakistani army official said there were strong indications that the US was ready to launch bombing raids against suspected al-Qaeda and Taleban camps inside Pakistan.

The official said that any unilateral American military action could have serious repercussions and create difficulties for Pakistani counter-terrorism efforts.

Washington is concerned by the new Government's move to strike peace deals with militant groups, pacts that American critics say will simply give insurgents time to regroup and gain strength.

Analysts say that the failure of the new coalition Government led by the Pakistan People's Party to formulate a clear counter-insurgency policy has affected the military's efforts to curb cross-border infiltration and the rising influence of militants in Pakistan's North West Frontier Province.

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