DERA ISMAIL KHAN, Pakistan — 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.

Lawmakers are also frustrated by suspicions that Pakistan is aiding militants who use its territory to attack U.S. troops in Afghanistan — allegations Islamabad has rejected. There is also lingering resentment over the fact that Osama bin Laden was found hiding deep inside Pakistan.

But the U.S. cannot afford to turn its back on Pakistan entirely.

The U.S. and other NATO countries fighting in Afghanistan shipped about 30 percent of their non-lethal supplies through Pakistan before the attack in November that killed Pakistani troops. The coalition has had to compensate since then by using a far more expensive path through Russia and Central Asia.

The route through Pakistan will become even more important as the U.S. begins to withdraw a decade's worth of equipment in the process of pulling out most of its combat troops from Afghanistan by the end of 2014.

Pakistan is also seen as vital to negotiating a peace deal with the Afghan Taliban and their allies given the country's historical ties with the militants.

The Pakistani government is also keen to repair relations with the U.S., partly to receive over a billion dollars in American aid it needs to fill out its budget as it looks ahead to national elections scheduled for 2013. But patching up ties is politically sensitive in a country where anti-American sentiment is rampant.

The issue of drone strikes is also complicated by the fact that some elements within the Pakistani government and military are widely believed to have supported the attacks in the past. That cooperation has declined as relations between the two countries have deteriorated, but many analysts believe there is still some support within Pakistan's senior ranks.

The U.S. refuses to discuss the covert CIA program in detail in public, but officials have said in private that the strikes are a vital anti-terror tool and have killed many senior al-Qaida and Taliban commanders.


Associated Press writer Sebastian Abbot contributed to this report from Islamabad.