US president Joe Biden has said America will hunt down the attackers behind twin explosions at Kabul airport in Afghanistan.
In a televised address, he revealed he had asked the Pentagon to develop plans to strike back at them.
The two blasts killed a dozen American troops and wounded more, making it the worst day of casualties for US forces there in a decade.
“We will not forgive, we will not forget. We will hunt you down and make you pay,” he said in remarks at the White House.
Biden insisted US evacuations would continue. “We will not be deterred by terrorists…we will continue the evacuation.”
He gave no indication of a change in next 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.”
He described the US troops who were killed as “part of a great noble company of American heroes”.
He said his heart also ached for all the Afghan families who lost loved ones including small children.
The situation on the ground was still evolving and he was constantly being updated, Mr Biden added.
Twelve US troops were killed and 15 other members of the American military injured, US general Frank McKenzie had confirmed earlier, amid reports that 60 civilians had died.
The head of American Central Command said on Thursday that two suicide bombers assessed to be from Islamic State carried out the attack.
He told a news briefing at the Pentagon that the bombing was followed by a gunfight and that evacuations from Afghanistan are continuing.
He said about 1,000 US citizens are estimated to still be in Afghanistan.
The US death toll, announced Gen McKenzie made it the deadliest single incident for American forces in Afghanistan in a decade and one of the deadliest of its entire 20-year war in the country.
Afghan health officials were quoted as saying 60 civilians died, but it was not clear whether that was a complete count.
Earlier, it emerged that the attack on Thursday involved at least two powerful suicide bombs and a gunman.
The attack occurred amid a huge evacuation effort from Afghanistan, and close to where people hoping to flee Afghanistan amid the Taliban takeover were queueing for visa processing.
Meanwhile, the US Defense Department has announced that the number of US troops killed in the blasts at Kabul airport rose by one to 13 and the number of injured was 18.
“A thirteenth US service member has died from his wounds suffered as a result of the attack on Abbey Gate,” Central Command Spokesman Captain Bill Urban said in a statement.