Baku, may 23 (AZERTAC). A U.S. drone fired two missiles at a compound in northwest Pakistan on Wednesday, killing four suspected militants in an attack that comes as Washington is running out of patience with Islamabad`s refusal to reopen supply routes for NATO troops in Afghanistan.

U.S. drone strikes have complicated negotiations over the routes, which Pakistan closed six months ago in retaliation for U.S. airstrikes that killed 24 Pakistani soldiers along the Afghan border. Pakistan`s parliament demanded the strikes stop in the wake of the attack, but the U.S. has refused.

The latest strike took place in Datta Khel Kalai village in the North Waziristan tribal area, said Pakistani intelligence officials, speaking on condition of anonymity because they were not authorized to talk to the media.

Parliament also demanded an "unconditional apology" from the U.S. for killing its troops. The Obama administration has expressed regret but is not willing to tender an apology out of concern that it could open the president up to attacks by Republicans angry at Pakistan`s lack of cooperation on the Afghan war.

Despite these disagreements, Pakistan appeared close to reopening the supply routes last week, prompting NATO to invite President Asif Ali Zardari to a major summit held May 20-21 in Chicago. But negotiations have faltered on Pakistan`s demand for much higher transit fees, and the U.S. made its frustration clear at the summit.

President Barack Obama refused to meet one-on-one with Zardari and did not mention Pakistan in the list of countries he thanked in his speech Monday for helping get war supplies into Afghanistan.

On Tuesday, a Senate panel approved a foreign aid budget for next year that slashes U.S. assistance to Pakistan by more than half and threatens further reductions if it fails to open the NATO supply routes.