Drone strikes have been a growing source of tension between Islamabad and Washington.
Islamabad and the country's political parties regularly denounce the attacks as a violation of Pakistan's sovereignty, although the country's government is known to have supported some of the strikes in the past.
The tension has further complicated a relationship that Washington views as vital to fight al-Qaida and the Taliban, as well as negotiate peace in Afghanistan.
The land routes through Pakistan from the southern port city of Karachi have been key to getting supplies to NATO troops in Afghanistan.
They now increasingly are being used to ship equipment out of Afghanistan as the U.S. seeks to withdraw most of its troops from the country by the end of 2014.
