President Joe Biden vowed to keep up airstrikes against the Islamic extremist group whose suicide bombing at the Kabul airport killed scores of Afghans and 13 American service members. Another terror attack, he said, is highly likely this weekend as the US winds down its evacuation.

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 Biden's deadline for ending the evacuation on Tuesday.

After getting briefed on a US drone mission in eastern Afghanistan that the Pentagon said killed two members of the Islamic State group's Afghanistan affiliate early Saturday, Biden said the extremists can expect more.

"This strike was not the last," 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 executing 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 evacuation proceeded as tensions rose over the prospect of another IS attack.

"Our commanders informed me that an attack is highly likely in the next 24-36 hours," 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.

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 remains of troops killed in action overseas are usually flown back to the US via Dover Air Base in Delaware, where fallen troops' return to US soil is marked by a solemn movement known as the dignified transfer.

The White House on Saturday did not say if Biden would travel to Dover for the troops' return. Biden's press secretary, Jen Psaki, said shortly after the attack that the president would do everything he can to honor the sacrifice and the service of those killed.

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.