Fewer civilians have died in US drone strikes in Pakistan so far this year than at any other time in the last four years, a report said Monday.

Three to 24 civilians were reported killed by American CIA drones in Pakistan from January to June, according to the London-based Bureau of Investigative Journalism.

Reported civilian casualty rates have not been so low since the first half of 2008, when 12-21 civilians reportedly died under former US president George W Bush, it said.

It was also a marked decline on the 62-103 civilians reported killed by drone strikes in Pakistan in the first six months of 2011, the bureau added.

US drones target Taliban and Al-Qaeda fighters in Pakistan's semi-autonomous tribal belt on the Afghan border, where journalists and aid workers do not have independent access.

The programme is covert, but US officials have defended the attacks as a vital weapon in the war on terror, despite concerns from rights activists over civilian casualties.

The decline in casualties correlates to a decline in attacks as relations between Islamabad and Washington deteriorated since Osama bin Laden was killed in Pakistan in May 2011 and after US air strikes killed 24 Pakistani soldiers last November.

According to an AFP tally, 101 US drone strikes were reported in Pakistan in 2010, 64 in 2011 and only 24 so far this year.

Pakistan has also becoming increasingly vocal in its public opposition to the strikes.

It stopped NATO supplies travelling overland into Afghanistan after its soldiers were killed in November. It has yet to reopen the route, demanding a formal apology for the deaths.

According to the bureau, between 2,496 and 3,202 people have been reported killed by drones in Pakistan since 2004. Among them are 482-832 civilians, 175 of them children.

UN High Commissioner for Human Rights Navi Pillay last month called for a UN investigation into US drone strikes, questioning their legality and saying they kill innocent civilians.