President Joe Biden, his voice breaking with emotion, has vowed the United States will hunt down those responsible for twin explosions at the Kabul airport in Afghanistan.
He has asked the Pentagon to develop plans to strike back at them.
Biden spoke on Thursday hours after the blasts killed at least a dozen American troops and scores of civilians, the worst day of casualties for US forces there in a decade.
Islamic State Khorasan (ISIS-K), an affiliate of militants who previously battled US forces in Syria and Iraq, claimed responsibility for the attack.
"We will not forgive, we will not forget. We will hunt you down and make you pay," Biden said in remarks at the White House.
He promised US evacuations would continue. He gave no indication of a change in Tuesday's US pullout target.
"I have also ordered my commanders to develop operational plans to strike ISIS-K assets, leadership and facilities. We will respond with force and precision at our time, at the place we choose and the moment of our choosing," Biden said.
Biden appeared to be fighting back tears and his voice cracked with emotion as he talked about the American "heroes" who died.
"It's been a tough day," he said.
The president said he had told the US military that he would grant additional force if they needed it: "Whatever they need, if they need additional force, I will grant it."
Biden defended his handling of his most serious foreign policy crisis, saying ultimately it is his responsibility, while assigning some blame to his predecessor, Republican Donald Trump, for the 2020 agreement Trump negotiated with the Taliban.
"I bear responsibility for, fundamentally, all that's happened of late," Biden told reporters when asked if he was responsible for the events of the past two weeks.
He said he did not trust the Taliban but believed it was in the group's interest to let the evacuations continue.
Biden had been warning of the possibility of attacks before the blasts erupted at the Kabul airport.
"I know of no conflict, as a student of history, no conflict when a war was ending one side was able to guarantee that everyone who wanted to be extracted from that country was able to get out," he said.
Australian Associated Press
