Two United States passport holders arrested while trying to enter Pakistan are of Afghan origin, say the Pakistani authorities.

FBI agents are helping question the pair, who were among a group detained in the semi-autonomous tribal area of North Waziristan a week ago.

Pakistani Interior Secretary Tasneem Noorani told Reuters they had entered Pakistan from Afghanistan without visas.

Officials say the group was trying to cross into Pakistan, but have given no further information.

They say it is not clear if there is a link with Osama Bin Laden's al-Qaeda network, blamed for the 11 September US attacks.

Fears that US nationals may be among al-Qaeda's members have grown in recent weeks.

Last month, an American citizen, Abdullah al-Muhajir, who had allegedly trained in Afghanistan, was arrested in Chicago, on suspicion of planning to explode a so-called "dirty" bomb.

Security concerns

Pakistan's tribal areas, set up after partition from India, stretch for hundreds of miles along the border with Afghanistan.

Although strictly speaking part of Pakistan, they have their own laws and customs and the writ of the authorities does not run there.

Pakistan has stepped up security along its border with Afghanistan to try and stop al-Qaeda and Taleban suspects coming into the country.

The US has been particularly concerned that this operation could be jeopardised by tension on Pakistan's other border with India.