Pakistan's ambassador to Afghanistan has gone missing before he was due to cross into Afghanistan from Pakistan.
Sources have told the BBC that the ambassador, Tariq Azizuddin, was kidnapped in the Khyber tribal agency close to the Afghan border.
The Pakistani embassy in Kabul says contact was lost with Mr Azizuddin at around 1045 local time (0645 GMT).
Many areas in the border regions between Afghanistan and Pakistan are strongholds of pro-Taleban militants.
Mr Azizuddin was going to Kabul from Peshawar by road when he disappeared.
The BBC's Alastair Leithead in Afghanistan says that police in Pakistan's Khyber Agency said they believed that Mr Azizuddin had been kidnapped.
There were reports on Pakistani television of his car going through a checkpoint without stopping.
Hundreds of people have been kidnapped in the dangerous border region in recent years - four Pakistani Red Cross workers went missing in the same area a few days ago.
An official of the Khyber agency tribal administration told the BBC that the ambassador went through the Khyber agency without taking a security escort that was waiting for him at the start of the tribal territory.
Correspondents say that such escorts are routinely sent with dignitaries and officials when they travel through tribal areas.
But some travellers dispense with them because they think it makes their movements more noticeable.
Mr Azizuddin is said to have previously travelled to Kabul by road, often without the tribal security escort.
The route through the agency is believed to be the shortest and quickest way between Peshawar, the Pakistani border city in North West Frontier Province (NWFP) and Kabul.
Being the main trade route, the Khyber agency road is busy in daylight hours, and is a major route supplying reinforcements and supplies to the US and Nato forces in Afghanistan.
The border areas are militant strongholds
It is also one of the most protected of all the tribal roads, with a contingent of tribal police posted every 100m. The paramilitary Frontier Corps have a fort along the road.
Our correspondent says that while few well known militant groups operate in the area, a local extremist cleric has occasionally created trouble.
But he has no links with any militant groups affiliated with the Taleban or al-Qaeda.
Security officials told the Reuters news agency that the envoy was due to have changed cars at the border, but did not reach the frontier.
The Pakistani foreign office said that it could not confirm or deny that a kidnapping had taken place.
Correspondents say the disappearance highlights continuing instability in nuclear-armed Pakistan - a key ally in the US-led "war on terror" - with important parliamentary polls just one week away.