US drone strikes in Pakistan have a "minimal" impact on militant recruitment, a respected think tank said Tuesday, contrary to arguments the controversial programme creates more extremists than it kills.

The Brussels-based International Crisis Group (ICG) said however the missile strikes in Pakistan's lawless tribal zones were not a long-term solution to militancy and urged Washington to be more open about the clandestine campaign.

Washington regards the drone campaign, which has killed up to 3,587 people since 2004 according to the Britain's Bureau of Investigative Journalism, as a valuable tool in the fight against Taliban and Al-Qaeda.

The missile strikes have also reportedly killed hundreds of civilians and opponents say this acts as a recruiting tool for militant outfits, but the ICG report published Tuesday downplayed this argument.

"The actual benefit to extremist groups, including in terms of recruitment, appears, however, minimal," the report said.

The "Drones: Myths and Reality in Pakistan" paper said the main causes of militancy in the seven tribal districts along the Afghan border were domestic.

"These include the absence of the state and insecurity due to the resulting political, legal and economic vacuum and the military's support of, provision of sanctuaries to, and peace deals with militant groups," it said.

The new government of Nawaz Sharif should base its counter-terror strategy around bringing the tribal regions into the legal and constitutional mainstream, ICG said.

Harsh laws dating back to the British colonial era which still apply in the tribal areas should be repealed, the report urged, and bringing violent extremists to justice within the Pakistani legal system would remove Washington's perceived need to carry out drone strikes in the tribal belt.

Washington should develop a more rigorous framework for drone attacks, the ICG said, to ensure they complied with international law, and transfer the programme from the CIA to the Defense Department.

The ICG said the report was primarily based on interviews in Pakistan with "stakeholders in the legal, political and NGO communities, as well as activists, journalists and researchers".

Sharif, who emerged victor from the May 11 general election, has called the drone strikes a "challenge" to his country's sovereignty and said Washington must take Pakistani concerns about them seriously.