The diplomat's disappearance on Monday highlighted the spiralling insecurity ahead of crucial elections next week in the nuclear-armed nation, a key ally in United States efforts against Islamic extremism.

The envoy, Mr Tariq Azizuddin, was heading to the Afghan capital Kabul with his driver when they disappeared in the Khyber tribal district, a lawless north-western region bordering Afghanistan, the foreign office said.

'He has gone missing, we are confirming he is missing but at this stage I cannot give you any more details,' foreign office spokesman Mohammad Sadiq said.

Security officials said tribal authorities were on Tuesday scouring the rugged area, the site of the famed Khyber Pass linking Afghanistan and Pakistan, and had closed the main road between the two countries.

The Pakistan embassy in Kabul said it last had contact with the ambassador on Monday morning as he travelled from the north-western city of Peshawar to Kabul. The Afghan foreign ministry said he was not in Afghanistan.

'We are trying our best to find out what happened,' embassy spokesman Naheem Khan said.

If confirmed, Mr Azizuddin would be the most senior of several government officials to have been abducted in the mountainous tribal belt. Blame has either fallen on Islamist militants or criminal kidnap gangs.

Officials said security forces had seen the envoy's car driven at speed through a checkpost with 'local people sitting in the front seat'. They raised the alarm when he did not reach the main post on the frontier.

The ambassador went missing in an area where two Pakistani employees of the International Committee of the Red Cross disappeared earlier this month. Their fate is unknown.

Pakistan's tribal zone has been wracked by fighting between government forces and Islamist militants linked to Al-Qaeda and the Taleban, although Khyber has been one of the more peaceful regions.

Militants had kidnapped around 250 Pakistani soldiers in the tribal zone of South Waziristan. They were reportedly released in exchange for several rebels held by Pakistani authorities.

Meanwhile, Pakistani officials were giving medical treatment to a senior figure in Afghanistan's Taleban movement who was captured near the south-western border between the two countries on Monday.

Mullah Mansoor Dadullah, the brother of the Islamist militia's slain military chief in Afghanistan, was captured along with at least five other militants in a gunbattle on Monday.

Dadullah had been responsible for operations against the North Atlantic Treaty Organisation and United States-led troops in the southern Afghan province of Helmand.

The Taleban said in December that he had been sacked for disobedience - a claim Dadullah's spokesman denied.

State media quoted Interior Ministry spokesman Brigadier Javed Cheema as saying that Dadullah was 'injured but out of danger'.

Islamist fighters in Pakistan who are variously described by officials as Al-Qaeda or pro-Taleban militants have been blamed for a string of suicide bombings and clashes with security forces in recent months.

Pakistani President Pervez Musharraf has accused a key militant leader, Baitullah Mehsud, of orchestrating December's assassination of opposition leader Benazir Bhutto.

Her killing caused the postponement of general elections due in January.

The polls are now scheduled for Monday but the run-up has been hit by further violence, including a suicide bombing in the tribal region of North Waziristan that targeted political activists going to an election rally.

Ms Bhutto's party was due to release her posthumously published autobiography in Islamabad later on Tuesday. -- AFP