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ISLAMABAD — The U.S. government's use of drone strikes came under mounting pressure on Friday, as Pakistan joined a chorus of criticism over the policy following the accidental killings of two al Qaeda hostages.

The White House said it was conducting "a thorough independent review to understand fully what happened and how we can prevent this type of tragic incident in the future" after it emerged that U.S. aid worker Warren Weinstein and Italian national Giovanni Lo Porto were killed by a CIA drone strike near the Pakistan-Afghanistan border.

While sending “heartfelt condolences” to Weinstein and Porto's relatives, Pakistan questioned the continued use of the technology in the region which has long been a source of tensions.

The country's Foreign Ministry said in a statement that the deaths underscored the risk and unintended consequences of the use of this technology that Pakistan has been highlighting for a long time.”

“Drones do not differentiate between terrorists and innocent civilians," Tasnim Aslam, a spokesperson for the country’s foreign ministry, later told NBC News. “We have said this before and still stand by it.”

The White House's confirmation on Wednesday that a U.S. drone killed the two hostages sparked immediate concerns from rights groups.

The U.K.-based Reprieve said "a fundamental reassessment of whether the secret drone war does more harm than good" was needed in wake of the incident.

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"It's right that the White House has come clean and admitted its tragic mistake in killing these hostages, and our hearts go out to their families." the group said in a statement. "It's worth remembering, however, that Dr. Weinstein and Mr. Lo Porto are far from the first innocents to die by our drones."

U.S. officials told NBC News that the hostages were killed in a controversial tactic known as a signature strike — when the CIA attacks based on a pattern of behavior of people on the ground, even if it doesn't know who it's targeting.

The deaths of Weinstein and Porto were announced along with news that drone strikes had killed two al Qaeda members who were U.S. nationals. The four deaths bring to seven the number of Americans killed in drone strikes in the last five years; only one of them was actually targeted.

Justin Bronk, an aerospace technology expert at the London-based Royal United Services Institute (RUSI), said the CIA "clearly did not have the assets on the ground to determine whether there were hostages there or not."

“The reason, in my opinion, that you can’t kill your way out of a situation with drones is because you will inevitably end up killing civilians and that will cause more and more resentment and lead to more radicalization,” he said.

The American Civil Liberties Union echoed Bronk's sentiments, saying in a statement Thursday that the incident raised "troubling questions about the reliability of the intelligence that the government is relying on to justify drone strikes.

"The U.S. quite literally didn't know who it was killing"

"In each of the operations acknowledged today, the U.S. quite literally didn't know who it was killing," ACLU Deputy Legal Director Jameel Jaffer said.

The use of covert drone strikes has long been criticized by the countries where they are deployed, even uniting Pakistan and Afghanistan, who have both called on the U.S. to stop using them.

A 2014 report by the United Nations Human Rights Council also called for independent investigations to be carried out into drone strikes following reports of civilian deaths in Afghanistan, Pakistan, Yemen and elsewhere.

However, the U.S. administration and President Barack Obama have repeatedly defended their use.

In a 2013 speech, Obama said that they were the most efficient and least bloody means of going after terrorists who posed an imminent danger, in remote and lawless places beyond the reach of government.

Drones had caused "heartbreaking" civilian casualties, he admitted, but they were safer than the alternatives, when an armed intervention by U.S. troops on the ground could cause both more deaths among innocent civilians and also trigger an international crisis.

— NBC News' Henry Austin reported from London.

Tracy Connor, Robert Windrem and Jim Miklaszewski contributed to this report.