Mr Khan's whereabouts are not known
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The head of the BBC World Service has called on the Pakistani government to help locate the missing BBC Urdu service reporter Dilawar Khan Wazir.
Mr Khan has not been heard from since Monday morning when he was due to leave Islamabad for his home in North-West Frontier Province (NWFP).
Police on Tuesday filed a criminal case against "unknown kidnappers".
The interior minister says that he cannot confirm whether Mr Khan is being detained in government custody.
'Suspicious circumstances'
"I am seriously concerned about the whereabouts of our reporter and would request your kind help in tracing him at the earliest," BBC World Service Director Nigel Chapman wrote in a letter to the Pakistani interior ministry.
The BBC said earlier that Mr Khan had disappeared in "suspicious circumstances".
Interior Minister Aftab Ahmed Sherpao told the BBC: "We are looking into the case and so far we are unable to find him.
"I am not in a position to confirm whether or not he is in government custody."
The NWFP regional assembly has meanwhile unanimously called on the central government to use all available resources to find the missing journalist.
A senior Islamabad police officer said that Mr Khan's last movements before his disappearance were now being investigated.
Fears for his safety were raised when his mobile phone was answered by someone saying he was seriously hurt in hospital. There was no sign of him when the BBC checked.
Pakistan's record on press safety is poor and Mr Khan has received threats. In August his young brother was killed.
It is not clear if that attack was linked to Mr Khan's work as a journalist.
He is one of the few local journalists reporting on the Pakistani army's fight with pro-Taleban militants in the troubled Waziristan region on the Afghan border.
A number of journalists have gone missing, and some have been killed, after covering stories considered sensitive by the military or the militants.
'Seriously injured'
Mr Khan had met another brother, Zulfiqar Ali, at the Islamic University in the capital on Sunday.
Police took a witness statement from Mr Ali on Tuesday.
He said that his brother was due to travel back to the town of Dera Ismail Khan where he lives on Monday but never arrived.
His family say a number of unidentified men came to the university hostel looking for his brother to tell him that Dilawar had been injured. He refused to accompany them.
When Zulfiqar Ali rang his brother's mobile phone, a man who gave his name as Dr Jamshed said Mr Khan had been seriously injured in a road accident and was in the Pakistani Institute of Medical Sciences (PIMS) in Islamabad.
A BBC reporter who went to the PIMS could find no trace of either Mr Khan or a Dr Jamshed.
BBC Urdu service head Mohammad Hanif said he was very worried for Mr Khan's safety.
"Considering the fact that we have been regularly reporting stories about journalists being picked up by security agencies in Pakistan, we are really concerned," he said.
"Some of these journalists remained missing for months and after their release told us that they were held by intelligence agencies in illegal custody and tortured."
Brother killed
Mr Khan and his family have been targeted on a number of occasions in recent years - it is not clear by whom.
Officials still do not know who abducted his 15-year-old brother, Taimur, in August. He was found with severe head wounds in the town of Wana in South Waziristan and later died of his injuries.
Last year, bombers targeted the Khans' house in Wana and a school run by his family. No one was hurt but the explosion damaged part of their house wall.
Mr Khan has said his family have no personal or tribal enemies.
In February 2005 two journalists in the same car as him were killed when shots were fired at their vehicle in Wana. He was unhurt.
They had been reporting on the signing of a peace agreement between the authorities and tribal fighters.
Mr Khan left his home in Wana last year and moved to Dera Ismail Khan after receiving threats from the militants.