Only 300 American citizens still in Afghanistan are seeking to leave the country, US Secretary of State Antony Blinken said Sunday, just days ahead of the US deadline for evacuations.

"We have about 300 American citizens left, who have indicated to us that they want to leave. We are very actively working to help them get to the airport, get on a plane and get out of Afghanistan," he told ABC.

Nearly 5,500 Americans have been evacuated as part of a mammoth operation that has flown more than 114,000 people from the country since the Taliban takeover.

Some Americans, National Security Advisor Jake Sullivan told Fox News Sunday, had chosen to stay beyond the August 31 deadline set to complete the evacuation, but he said "they are not going to be stuck in Afghanistan."

The US has "a mechanism to get them out," Sullivan added, without elaborating.

Seeking to reassure Americans and their remaining allies in Afghanistan about the deeply uncertain future, he said "The Taliban have made commitments to us...and we have leverage to hold them to those commitments."

Sullivan did not elaborate about the leverage, but the United States and other countries have frozen billions of dollars of Afghan government assets.

US officials have warned about continuing danger around Kabul international airport and urged people to stay away.

An attack there Thursday claimed dozens of lives, including those of 13 American service members. US President Joe Biden and his wife, Jill, were at Dover Air Force Base in Delaware on Sunday to meet the plane returning their remains and to offer condolences to family members.

But officials warned that the final winding down of the United States' 20-year presence in Afghanistan could hold some of the highest risks.

"This is the most dangerous time in an already extraordinarily dangerous mission," Blinken said.

Sullivan told Fox News Sunday that "another attack could occur at any time."