As the United States resumed drone strikes inside Pakistan ignoring Islamabad's warnings, US President Barack Obama's top counterterrorism adviser defended the use of armed unmanned aircraft to target suspected terrorists overseas.
The strikes conducted "in full accordance with the law" are used when the option of capture is not feasible, deputy national security adviser said during a address on Monday at the Woodrow Wilson Centre, a Washington think-tank.
"President Obama said here five years ago, if another nation cannot or will not take action, we will," John Brennan said.
"And it is an unfortunate fact that to save many innocent lives we are sometimes obliged to take lives - the lives of terrorists who seek to murder our fellow citizens."
Brennan said the United States "respects national sovereignty and international law" and is guided by the laws of war in ordering those attacks.
His remarks came a day after a drone strike hit a high school in Pakistan's northwestern tribal region where intelligence officials said Islamic militants were hiding Sunday, bringing a fresh denunciation from Pakistan's Ministry of Foreign Affairs.
Brennan said the targets are usually people holding high-ranking positions within al Qaeda, or who possess critical skills that help enable the terrorist organisation to carry out attacks, and there is no feasible chance to capture them.
The programme is now spreading beyond the United States, he said voicing for the first time the administration's concerns about the possibility of other countries using this technology against the United States.
"President Obama and those of us on his national security team are very mindful that as our nation uses this technology, we are establishing precedents that other nations may follow," he said.
"And not all of them will be nations that share our interests or the premium we put on protecting human life, including innocent civilians."