Updated

A suspected U.S. drone fired two missiles at a house in northwest Pakistan early Friday morning, killing four militants in an attack that comes as Pakistani officials have stepped up their calls for the strikes to end, intelligence officials said.

The attack could complicate U.S. efforts to get Pakistan to reopen its border crossings to supplies meant for NATO troops in Afghanistan. Pakistan shut the border last November in retaliation for American airstrikes that killed 24 Pakistani soldiers.

Pakistan's parliament is debating a revised framework for its relationship with the U.S. that Washington hopes will result in NATO supply routes reopening. But a key demand is that the U.S. stop drone attacks, which are very unpopular in Pakistan because many people believe they mostly kill civilians -- a claim denied by the U.S. and contradicted by independent research.

Friday's strike targeted a house in Miran Shah, the main town in the North Waziristan tribal area, a key sanctuary for Taliban and Al Qaedamilitants, Pakistani intelligence officials said, speaking on condition of anonymity because they were not authorized to talk to the media.

The four militants killed in the strike and three wounded were from Uzbekistan, said the officials. Their precise identities were unclear. The attack occurred while the group was sleeping, the officials said.

The U.S. rarely talks publicly about the covert CIA-run drone program in Pakistan, but officials have said privately that the strikes are a key component of America's war against Islamist militants and have killed senior Taliban and Al Qaedacommanders.

President Barack Obama stepped up the drone campaign in Pakistan when he took office in 2009, and most of the attacks have targeted North Waziristan. But the strikes have dropped off significantly in recent months as the relationship between the U.S. and Pakistan has deteriorated.

Pakistani officials have regularly criticized the attacks as a violation of the country's sovereignty. But the military was widely believed to help with at least some of the strikes and allowed the drones to take off from bases inside Pakistan.

That cooperation has come under serious strain as ties have worsened, especially following the deaths of the Pakistani soldiers at two posts along the Afghan border. In addition to cutting off NATO supplies, Pakistan kicked the U.S. out of a base used by American drones in the country's southwest.

The move was not expected to significantly impact the drone program since the base was only used to repair aircraft that took off from Afghanistan. But it did signal the government's growing opposition to the drones, culminating in the parliamentary debate currently taking place.

One of the reasons the U.S. has relied so heavily on drone strikes in Pakistan, and is reluctant to give them up, is that Islamabad has rejected calls to target militants in North Waziristan who are using the area as a base to attack American troops in Afghanistan.

The most important group is the Haqqani network, considered the most dangerous militant faction fighting the U.S. in Afghanistan.

Pakistan has argued that it can't conduct an offensive in North Waziristan because its troops are stretched too thin by operations against militants within the country who threaten its own government.

But many analysts believe Pakistan is reluctant to target the Haqqani network and its allies in the Afghan Taliban because they are seen as important allies in Afghanistan after foreign forces withdraw.

Pakistan has close historical ties with both groups, and U.S. officials have accused the country's shadowy spy agency, the ISI, of continuing to provide them with support -- an allegation denied by Islamabad.

Analysts argue that Pakistan's calculus has not changed despite billions of dollars in U.S. military and civilian aid over the last decade meant to enlist greater support in the fight against Islamist militants.