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.
Mr 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.
A source familiar with US congressional briefings had said US officials strongly believe that the Afghan affiliate of Islamic State, known as Islamic State Khorasan (Isis-K), after an old name for the region, was responsible.
A Taliban official said at least 13 people had been killed, including children, and 52 wounded, including many Taliban guards.
An emergency hospital in Kabul said it had received 30 wounded following the explosions, with six dying on the way to hospital.
Video images uploaded by Afghan journalists showed dozens of bodies of people killed in tightly packed crowds outside the airport. A watery ditch by the airport fence was filled with bloodsoaked corpses, some being fished out and laid in heaps on the canalside while wailing civilians searched for loved ones.
Islamic State, which has emerged in Afghanistan as enemies both of the West and the Taliban, claimed responsibility in a statement in which it said one of its suicide bombers targeted “translators and collaborators with the American army”.
Pentagon spokesman John Kirby said one blast occurred near the airport’s Abbey Gate and the other close to the nearby Baron Hotel.
“We can confirm that the explosion at the Abbey Gate was the result of a complex attack that resulted in a number of US & civilian casualties,” Mr Kirby said on Twitter. “We can also confirm at least one other explosion at or near the Baron Hotel, a short distance from Abbey Gate.”
The US embassy in Kabul described “a large explosion” near the airport and said there had been reports of gunfire.
Following the attack the Taliban blamed the US, which is directing a military-led evacuation from Kabul, for drawing large crowds to the airport perimeter.
There was no indication from the White House on Thursday that US president Joe Biden plans to change an August 31st withdrawal deadline as a result of the explosions, a source familiar with the situation told Reuters.
The attacks occurred about the time Mr Biden was scheduled to meet with his national security team about the situation in Afghanistan. He has since been briefed in the White House Situation Room, according to an official.
“Bodies, flesh and people were thrown into a canal nearby,” said one witness who was at the scene of the first blast. “When people heard the explosion there was total panic.”
“The Taliban then started firing in the air to disperse the crowd at the gate,” a second witness said. “I saw a man rushing with an injured baby in his hands.”
The US and allies had urged people to move away from Kabul airport on Thursday due to the threat of an Islamic State terror attack as western troops hurry to evacuate as many Afghans as possible before the August 31st deadline.
Pressure to complete the evacuations of thousands of foreigners and Afghans who helped western countries during the 20-year war against the Taliban has intensified, with all US and allied troops due to leave the airport next week.
In an alert issued on Wednesday evening, the US embassy in Kabul advised citizens to avoid travelling to the airport and said those already at the gates should leave immediately, citing unspecified “security threats”.
In a similar advisory, Britain told people in the airport area to “move away to a safe location”.
“There is an ongoing and high threat of terrorist attack,” the British foreign office said in its statement.
Australia also urged its citizens and visa holders to leave the area, warning of a "very high threat of a terrorist attack" at the airport.
The warnings came against a chaotic backdrop in Kabul and its airport, where the massive airlift has been under way since the Taliban captured the city on August 15th.
While western troops in the airport worked feverishly to move the evacuation as fast as possible, Taliban fighters guarded the perimeter outside, thronged by thousands of people trying to flee rather than stay in a Taliban-ruled Afghanistan.
Ahmedullah Rafiqzai, an Afghan civil aviation official working at the airport, said people continued to crowd around the gates despite the attack warnings.
“It’s very easy for a suicide bomber to attack the corridors filled with people and warnings have been issued repeatedly,” he told Reuters.
“But people don’t want to move, it’s their determination to leave this country that they are not scared to even die, everyone is risking their lives.”
Taliban guards continue to protect civilians outside the airport, an official of the Islamist group said earlier on Thursday.
“Our guards are also risking their lives at Kabul airport, they face a threat too from the Islamic State group,” said the official, who spoke on condition of anonymity.
The White House said Mr Biden was briefed on Wednesday about the threat from the Isis-K militant group, as well as contingency plans for the evacuation.
Mr Biden has ordered all troops out of Afghanistan by the end of the month, to comply with an agreement with the Taliban, despite European allies saying they needed more time to get people out.
In the 11 days since the Taliban swept into Kabul, the US and its allies have mounted one of the biggest air evacuations in history, bringing out almost 100,000 people. The US military says planes are taking off the equivalent of every 39 minutes.
US secretary of state Antony Blinken said at least 4,500 American citizens and their families had been evacuated from Afghanistan since mid-August.
The Taliban have said foreign troops must be out by the end of the month. They have encouraged Afghans to stay, while saying those with permission to leave will still be allowed to do so once commercial flights resume after the foreign troops go.
The militant group has asked Nato member Turkey to help keep the airport open after foreign troops leave. Turkey said technical experts might remain to help operate the airport.
The UN is leaving some 3,000 Afghan staff at its mission. A UN security document reviewed by Reuters described dozens of incidents of threats, the looting of UN offices and physical abuse of staff since August 10th.
The Taliban’s 1996-2001 rule was marked by public executions and the curtailment of basic freedoms. Women were barred from school or work.
The US-backed Afghan government collapsed swiftly after Mr Biden moved to withdraw troops from Afghanistan, two decades after US-backed forces had ousted the Taliban in the weeks following the September 11th, 2001, attacks on the US, planned from Afghan territory by al-Qaeda. – Reuters/Guardian/AP/Bloomberg