US President Joe Biden has warned that another attack on Kabul airport is “highly likely in the next 24-36 hours”, while the State Department called the threat “specific” and “credible”.
Mr Biden also vowed Saturday to continue airstrikes against the ISIS-K group whose suicide bombing at Hamid Karzai International Airport on Thursday killed 170 people and injured dozens more.
The Pentagon said the remaining contingent of US forces at the airport, now numbering fewer than 4,000, had begun their final withdrawal ahead of Mr Biden's deadline for ending the evacuation on Tuesday.
US intelligence is warning of an 'imminent attack' at Kabul airport in the next few days, US Correspondent Emma Murphy says
After a briefing on a US drone mission in eastern Afghanistan that the Pentagon said killed two members of the Islamic State group's Afghanistan affiliate early on Saturday, Mr Biden said the extremists can expect more.
“This strike was not the last,” Mr Biden said in a statement.
“We will continue to hunt down any person involved in that heinous attack and make them pay”.
He paid tribute to the “bravery and selflessness” of the American troops carrying out the hurried airlift of tens of thousands from Kabul airport, including the 13 US service members who were killed in Thursday's suicide bombing at an airport gate.
The State Department issued a new security alert early on Sunday morning Kabul time, instructing people to leave the airport area immediately “due to a specific, credible threat”, as tensions rose over the continuing and urgent evacuation process.
“Our commanders informed me that an attack is highly likely in the next 24-36 hours,” Mr Biden said, adding that he has instructed them to take all possible measures to protect their troops, who are securing the airport and helping bring onto the airfield Americans and others desperate to escape Taliban rule.
As US forces prepare to leave Afghanistan by Tuesday, the last UK military and diplomatic personnel landed back in the UK on Sunday morning, marking the end of the UK's 20-year campaign in Afghanistan.
Operation Pitting, the largest evacuation mission since the Second World War, ended on Saturday night as the final troops left.
The Ministry of Defence said the last civilian evacuation flight left Kabul earlier on Saturday.
More than 15,000 people have been airlifted to safety in just over a fortnight, as more than 1,000 troops, diplomats, and officials were sent to Afghanistan to rescue British nationals and Afghan allies after the Taliban took over the country's capital.
But hundreds of Afghans who helped the UK during the war have been left behind and face the dangers of Taliban rule.
Prime Minister Boris Johnson said now was “a moment to reflect on everything we have sacrificed and everything we have achieved in the last two decades”.
It comes as the remains of the 13 American troops were on their way to the United States, the Pentagon said.
Their voyage marked a painful moment in a nearly 20-year American war that cost more than 2,400 US military lives and is ending with the return to power of a Taliban movement that was ousted when US forces invaded in October 2001.
The Pentagon released the names of those killed — 11 Marines, one Navy sailor and one Army soldier. Twelve of them were in the 20s; some were born in 2001, the year America's longest war began. The oldest was 31.
They were the first US service members killed in Afghanistan since February 2020, the month the Trump administration struck an agreement with the Taliban in which the militant group halted attacks on Americans in exchange for a US agreement to remove all troops and contractors by May 2021.
Mr Biden announced in April that the 2,500 to 3,000 troops who remained would be out by September, ending what he has called America's forever war.
With Mr Biden's approval, the Pentagon this month sent thousands of additional troops to Kabul airport to provide security and to facilitate the State Department's chaotic effort to evacuate thousands of Americans and tens of thousands of Afghans who had helped the United States during the war.
The evacuation was marred by confusion and chaos as the US government was caught by surprise when the Afghan army collapsed and the Taliban swept to power August 15.
Untold numbers of vulnerable Afghans, fearful of a return to the brutality of pre-2001 Taliban rule, are likely to be left behind.
Mr Biden and the leaders of other Western countries have said they would try to work with the Taliban to allow Afghans who had worked with them to leave after the US-led evacuation ends.
The Pentagon said that about 6,800 people, mostly Afghans, were flown out in the 24 hours that ended on Saturday morning, bringing to 113,500 the total number of people of all nationalities evacuated since the hurried exit was begun August 14.