Thirteen US service members and at least 60 Afghans have been killed in two bombing attacks outside Kabul's airport, according to the Pentagon and Afghanistan's Ministry of Public Health.
The deadly blasts came as the United States and other Western countries raced to complete a massive evacuation of their citizens and Afghan allies following the Taliban takeover of the country.
An official with Afghanistan's Ministry of Public Health told CNN on Thursday that more than 60 Afghan people were dead and 140 wounded.
Eighteen US service members were injured in addition to the 13 dead, said Capt. Bill Urban of US Central Command (Centcom).
US President Joe Biden, speaking from the White House, called the troops "heroes" and said he was "outraged as well as heartbroken."
"We will not forgive. We will not forget. We will hunt you down and make you pay," he warned the perpetrators of the attack.
ISIS in Khorasan, known as ISIS-K, has claimed that an ISIS militant carried out the suicide attack, but provided no evidence to support the claim.
US officials have said the group was likely behind the attack, and Biden on Thursday announced that he had ordered US military commanders "to develop operational plans to strike ISIS-K assets, leadership and facilities."
In previous days, the President had cited the risk of a terror attack among the reasons for getting US troops out of the country by August 31. He had also promised a swift and forceful response to any disruption to the operation.
US officials have been warning over the past week that a threat of a terror attack at the airport was becoming more acute. Earlier on Thursday local time, US diplomats in Kabul warned American citizens to "immediately" leave several gates into the airport, citing security threats.
The risk of potential suicide attacks by ISIS-K had already led the US to establish alternative routes to Kabul airport, earlier on in the evacuation operation.
Biden said he didn't feel that it had been a mistake to rely on the Taliban to provide security outside the airport.
The President said that while he does not trust the Taliban, it was in the best interests of both the United States and the Taliban to try and prevent an attack from ISIS.
Thousands of Afghans have been gathering at the airport's gates in recent days in hopes of being evacuated. Footage posted to social media on Thursday after the explosions showed chaotic scenes of crowds of people trying to help the wounded amid bodies on the ground. Photos showed apparently injured people being transported away from the scene in wheelbarrows.
Ten Marines were among the troops killed and several more were wounded, Marine spokesman Maj. Jim Stenger said. The identities and units of those killed won't be announced until Friday after relatives are notified, he added.
Pentagon spokesman John Kirby said earlier that one of the explosions happened "at the airport's Abbey Gate" and "at least one other explosion happened at or near the Baron Hotel, a short distance from Abbey Gate."
Abbey Gate has become the main entry point to the airport and primary security there has been provided by US Marines. The area around that gate had been used for holding refugees after they passed through the Taliban check points outside the airport, and before they were allowed to go to the airport.
Baron Hotel was used by British soldiers and other allies as an evacuation handling center to process evacuees, before moving them up to the Abbey Gate. It is unclear whether international forces were still in the area when the explosion happened.
The US Embassy in Kabul said US citizens who were at the Abbey Gate, East Gate, or North Gate of the airport "should leave immediately" in the wake of the blast.
"There has been a large explosion at the airport, and there are reports of gunfire," the security alert said. "US citizens should avoid traveling to the airport and avoid airport gates at this time," it added.
Immediately after the explosions, gunmen opened fire on service members and civilians, said Marine Gen. Frank McKenzie, Centcom Commander.
Hours after the attacks, coalition forces also conducted a series of scheduled and controlled explosions within Hamid Karzai International Airport, US Central Command spokesman Maj. John Rigsbee
Evacuations to continue, but winding down
Tens of thousands of people have so far been evacuated by the US military and NATO allies from the airport in the past two weeks. These evacuations are set to start winding down in the next few days, ahead of US' August 31 deadline for the final exit from a 20-year war in Afghanistan.
Over 95,700 people have been evacuated since August 14 and over 101,300 since the end of July. Even after the attack, scores of people continued to gather at the airport.
McKenzie said that the evacuations will continue despite the attack. "Our mission is to evacuate US citizens, third country nationals, special immigrant visa holders, US embassy staff, and Afghans at risk. Despite this attack, we are continuing the mission, the evacuation at best speed," he said.
"But right now our focus really, we have other active threat streams, extremely active threat streams against the airfield, we want to make sure we are taking the steps to protect ourselves there. Our focus is on that," he added.
He said the threats from ISIS were "imminent," raging from rocket attacks to "vehicle-borne" suicide attacks in addition to "walk-in" suicide attackers like the attack today.
McKenzie also said the US is sharing some intel with the Taliban for security purposes.
"They don't get the full range of information we have. But we give them enough to act in time and space to try to prevent these attacks," said McKenzie. He added that the US is using attack helicopters and other manned and unmanned aircraft to defend the airport in Kabul.
Biden said he would authorize whatever military leaders need, including more troops.
"These ISIS terrorists will not win. We will rescue the Americans. We will get our Afghan allies out. And our mission will go on," the US President said. "America will not be intimidated."