US secretary of defense Leon Panetta backed the use of drones to target terrorists in Pakistan amid a row over a missile strike that reportedly killed Al-Qaeda's number two leader.
Washington says Abu Yahya al-Libi was killed when two missiles struck a suspected terrorist compound early on Monday in Hesokhel, a village in the North Waziristan tribal area.
At least 14 people are believed to have been killed in addition to the primary target.
The US had made it "very clear that we are going to continue to defend ourselves," Panetta said while attending a conference in neighbouring India.
"This is about our sovereignty as well," he added.
On Tuesday, Pakistan summoned the US charge d'affaires to reiterate its "serious concerns" about drone strikes.
There have been eight US drone strikes in the past two weeks despite Islamabad's demands for them to be stopped.
Pakistan says the drone attacks fuel anti-US sentiment and claim civilian casualties along with militants. The US insists the strikes are effective.
The White House said Libi's death dealt a heavy blow to al-Qaeda as he played a critical role in its planning of operations against the West.
"There is no-one who even comes close in terms of replacing the expertise al-Qaeda has just lost," one official told the BBC.
Panetta argued that al-Qaeda leaders who had orchestrated the 11 September 2001 attacks were located in Pakistan's north-western Federally Administered Tribal Areas (Fata), which border Afghanistan.
He also said the drone strikes helped protect Pakistanis, who have been the targets of attacks by al-Qaeda and its allies.
"Pakistan is a complicated relationship, complicated for both of our countries, but it is one that we must continue to work to improve," Panetta added.