President Joe Biden vowed on Thursday to complete the evacuation of American citizens and others from Afghanistan despite the day’s deadly suicide bomb attack at the Kabul airport. He promised to avenge the deaths of 13 U.S. service members killed in the attack, declaring to the extremists responsible: “We will hunt you down and make you pay.”

Speaking with emotion from the White House, Biden said the Islamic State group’s Afghanistan affiliate was to blame for the attacks that killed the Americans and many more Afghan civilians. He said there was no evidence they colluded with the Taliban, who now control the country.

He asked for a moment of silence to honor the service members, bowing his head, and ordered U.S. flags to half-staff across the country.

As for the bombers and gunmen involved, he said, “We have some reason to believe we know who they are … not certain.” He said he had instructed military commanders to develop plans to strike IS “assets, leadership, and facilities.”

Gen. Frank McKenzie, the U.S. Central Command chief, said more attempted attacks were expected.

The IS affiliate in Afghanistan has carried out many attacks on civilian targets in the country in recent years. It is far more radical than the Taliban, who seized power less than two weeks ago. The most heralded American attack on the group came in April 2017 when the U.S. dropped the largest conventional bomb in its arsenal on an IS cave and tunnel complex. The group more recently is believed to have concentrated in urban areas, which could complicate U.S. efforts to target them without harming civilians.

“We will respond with force and precision at our time, at the place of our choosing,” Biden said. “These ISIS terrorists will not win. We will rescue the Americans; we will get our Afghan allies out, and our mission will go on. America will not be intimidated.”

Biden said U.S. military commanders in Afghanistan had told him it is important to complete the evacuation mission. “And we will,” he said. “We will not be deterred by terrorists.”

Indeed, Gen. McKenzie, who is overseeing the evacuation operation from his Florida headquarters, told a Pentagon news conference shortly before Biden spoke, “Let me be clear, while we are saddened by the loss of life, both U.S. and Afghan, we are continuing to execute the mission,.” He said there were about 5,000 evacuees on the airfield Thursday awaiting flights.

As many as 1,000 Americans and many more Afghans are still struggling to get out of Kabul.

McKenzie said 12 U.S. service members had been killed and 15 were wounded. Later, his spokesman, Capt. William Urban said the toll had risen to 13 dead and 18 wounded. Urban said the wounded were being evacuated from Afghanistan aboard Air Force C-17 transport planes equipped with surgical units.

The Marine Corps said 10 Marines were among those killed. Central Command did not identify the dead by service. It was the deadliest day for U.S. forces in Afghanistan since August 2011, when a helicopter was shot down by an insurgent armed with a rocket-propelled grenade, killing 30 American troops and eight Afghans.

In somber, sometimes halting remarks, Biden praised U.S. forces and asked for a moment of silence. Asked later about further actions, press secretary Jen Psaki said that personal calls to families would wait for notification of next of kin and that Biden might travel to Dover Air Force Base in Delaware when the remains of the fallen service members are returned.

They were the first U.S. service members killed in Afghanistan since February 2020, the month the Trump administration struck an agreement with the Taliban that called for the militant group to halt attacks on Americans in exchange for a U.S. agreement to remove all American troops and contractors by May 2021. Biden announced in April that he would have all forces out by September.