President Joe Biden was preparing Sunday at a US military base to receive the remains of the 13 American service members killed in an attack in Kabul, a solemn ritual that comes amid fierce criticism of his handling of the Afghanistan crisis.

Biden and his wife, Jill, both wearing black and with black face masks, first met far from the cameras with relatives of the dead in a special family centre at Dover Air Force Base in Delaware.

The base, on the US East Coast about two hours from Washington, is synonymous with the painful return of service members who have fallen in combat.

In a carefully scripted ceremony set for noon (16H00 GMT), a specially trained white-gloved crew will remove the "transfer cases" carrying the remains from the military cargo plane that has returned them to US soil, as officials salute and aggrieved family members watch.

Meeting the remains of fallen service members at Dover is considered one of the most solemn duties of a US president. For Biden - whose late son Beau Biden served in Iraq - the loss must have been particularly painful.

In addition to the 13 American service members, more than 100 Afghans died in the attack Thursday, blamed on a regional arm of the Islamic State. The US has since launched at least two drone strikes on Islamic State targets, including one Sunday against a vehicle said to be threatening the Kabul airport.

The chaotic US withdrawal from Afghanistan -- some 114,000 people have been flown out of the country since the Taliban takeover -- has also left Biden facing scorching criticism, particularly from Republicans.

About 48 hours from the US deadline for evacuations, Secretary of State Antony Blinken said that the United States was working tirelessly to evacuate Americans who want to leave.

Approximately 250 US nationals remain in Afghanistan, according to a State Department spokesperson. Some 280 others have self-identified as Americans but not yet decided whether they want to leave, the spokesperson said.

Those who chose to stay "are not going to be stuck in Afghanistan," National Security Advisor Jake Sullivan told Fox News Sunday.

Senator Ben Sasse, a Republican centrist -- he voted to impeach President Donald Trump -- lashed into the current president in an unusually blunt interview Sunday.

"Joe Biden put our forces at risk by having no plan for how to evacuate," said Sasse, who serves on the intelligence committee. "We are absolutely at risk. And we are at risk because the president has been so unbelievably weak."

He called an earlier interview with Blinken "disgusting." And Mitch McConnell, the Senate Republican leader, told Fox that Biden's withdrawal plan was "one of the worst foreign policy decisions in American history -- much worse than Saigon because after we left Saigon there weren't Vietnamese terrorists who were planning on attacking us here at home."