The missile-firing drones that the United States uses to strike suspected insurgents in Pakistan take off and land from a base inside that country, a US senator has revealed.

Senator Dianne Feinstein's disclosure was unusual because the US government has refused to acknowledge conducting missile strikes in Pakistan's tribal areas, a public source of tension between the two countries.

Feinstein, who heads the Senate Intelligence Committee, noted the apparent contradiction in Islamabad's position Thursday in a hearing with retired admiral Dennis Blair, the new director of national intelligence.

"I don't know whether you'd care to comment on this but (I) also noticed that Mr Holbrooke in Pakistan ran into considerable concern about the use of the Predator strikes in the FATA area of Pakistan.

"And yet, as I understand it, these are flown out of a Pakistani base," she said.

Richard Holbrooke, the special envoy for Pakistan and Afghanistan, was in Islamabad earlier this week as part of a tour of the region.

The CIA and the Pentagon refused to comment on Feinstein's disclosure.

The missile strikes have intensified in recent months amid stepped-up US efforts to target suspected Al-Qaeda and Taliban leaders in safe havens in the tribal areas.

The missiles are believed to have killed numerous militants, including key Al-Qaeda operatives, but also have claimed civilian casualties, according to Pakistani authorities, who have vehemently protested the violation of their sovereignty.

US and Pakistani media have reported that Islamabad and Washington have a secret agreement that authorizes the strikes.