Two suicide bombers and gunmen attacked crowds of Afghans flocking to Kabul's airport Thursday, trying to escape from the Taliban.
At least 60 Afghans and 13 U.S. troops were killed.
U.S. officials initially said 11 Marines and one Navy medic were among those who died. Another service member died hours later.
Another 18 service members were wounded and officials warned the toll could grow.
More than 140 Afghans were wounded, an Afghan official said.
US officials say an ISIS terrorist detonated a suicide vest at one of the airport gates while another opened fire. Then a second bomb went off outside a hotel about 300 yards away.
The first blast happened at what is known as the Abbey gate, where American citizens would line up to board evacuation flights.
The victims were standing outside the gate in a sewage canal when the explosion occurred.
Phone video of the blast aftermath showed piles of dead bodies.
Pentagon spokesman John Kirby tweeted "We can confirm that a number of US service members were killed in today's complex attack at Kabul airport. A number of others are being treated for wounds."
The Taliban have condemned the attack, saying it occurred in an area controlled by U.S. forces.
ISIS terrorism has been expected after large numbers of ISIS fighters were believed to have been freed from prisons during the Taliban's rapid advance.
The U.S. embassy in Kabul, which had an advanced word of a possible terrorist attack, warned Americans Wednesday to leave the airport gate "immediately" and to be aware of their surroundings, especially in large crowds.
Ross Wilson, acting U.S. ambassador to Afghanistan had warned, "Being part of these huge crowds that remain around the gates and entrances to the airport is dangerous. We're obviously concerned about our own people as well."
Wilson says Americans will be evacuated on an "individualized basis."
Secretary of State Antony Blinken says as many as 1,500 American citizens could still be in Afghanistan and insists those who want to leave will be able to.
"There is no deadline on our work to help any remaining American citizens who decide they want to leave to do so, along with the many Afghans who have stood by us over these many years," Blinken said.
But it's unclear how Americans or their Afghan allies will be evacuated after U.S. forces leave in less than a week.
And thousands of Afghans who helped the U.S. are now facing that grim reality that they could be left behind.