A Pakistani anti-drone campaigner, who was due to testify before European parliaments, has gone missing in the city of Rawalpindi, his lawyer said.

Kareem Khan, whose brother and teenage son were killed in a drone attack in December 2009, was picked up at his home by security forces in the early hours of February 5, Shahzad Akbar said on Monday, the Agence France Press news agency reported.

Akbar said 15 to 20 men, some wearing police uniform and others in plain clothes, seized Khan, who was the first Pakistani to sue his government over the drone attacks that resulted in the deaths of his relatives.

Khan has not been heard from since, Akbar said, accusing the country's intelligence agencies of illegal kidnapping.

"We lodged a report with the local police but they denied having picked him up," he said. "It seems to be work of the intelligence agencies."

The lawyer said no reason was given to Khan's wife and young children when he was detained.

A senior police official denied the force had arrested Khan.

"We had no 'raid' on record on that night," said the official, who requested anonymity because of the sensitivity surrounding the case.

'A crucial witness'

Khan had filed a case against the Pakistani government petitioning the court against drones strikes, arguing they constitute murder under domestic law.

The next hearing date before the Islamabad High Court is scheduled for Tuesday.

He was also due to travel to Europe on Saturday to speak with German, Dutch and British parliamentarians about his personal experience with drone strikes and the impact they are having on his country.

Akbar gained prominence after disclosing the identity of the then CIA station chief in Islamabad in 2010 in an application filed in a Pakistani court against him for his alleged role in drone strikes.

The case sparked loud criticism from Washington who accused Pakistan's intelligence agencies of leaking the name of the spy chief, who was later called back to Washington.

According to an AFP tally, 2,155 people have been killed in drone attacks since August 2008, with critics charging that the strikes cause many civilian casualties.