As many as 15,000 Americans remain in Afghanistan after the Taliban's takeover of the country, Biden administration officials told Senate staffers Tuesday, two aides said.
Two Senate aides confirmed that they were given that figure in a briefing led by national security and defense officials.
The Washington Post first reported that the staffers were told that 10,000 to 15,000 U.S. citizens remain in the country.
U.S. officials are racing to get Americans and others out of the country.
"The Taliban have informed us that they are prepared to provide the safe passage of civilians to the airport, and we intend to hold them to that commitment," national security adviser Jake Sullivan told reporters Tuesday at the White House.
"We are getting people through the gate, we are getting them lined up, and we are getting them on planes, but this is an hour-by-hour issue," he said.
Senate staffers at the briefing said they were told that the administration doesn't have a plan to evacuate U.S. citizens outside Kabul because of Taliban checkpoints, the two aides said.
The U.S. government said Monday that it was prepared to take more than 20,000 Afghans who are candidates for Special Immigrant Visas, or SIVs, with them to U.S. bases.
By Tuesday night, about 1,100 U.S. citizens, permanent residents and their families had been evacuated on 13 flights, U.S. officials said.
"Now that we have established the flow, we expect those numbers to escalate," the White House said in a statement, adding that in addition to the more than 3,200 Americans who have been evacuated, the U.S. has relocated nearly 2,000 Afghan special immigrants.
Afghanistan's military and government collapsed as U.S. forces withdrew after 20 years of conflict.
After Afghan forces surrendered to the Taliban and President Ashraf Ghani fled the country Sunday, throngs of people gathered at Kabul's international airport in a desperate bid to get out of the country.
The airport was temporarily shut down Monday because of the chaos, but flights resumed Monday night.
Hamid Karzai International Airport in Kabul remains open, and Marine Gen. Frank McKenzie, head of U.S. Central Command, said he had warned the Taliban against any interference.
"In meetings with Taliban senior leaders in Doha on Sunday, I cautioned them against interference in our evacuation and made it clear to them that any attack would be met with overwhelming force in the defense of our forces," McKenzie said.
President Joe Biden defended the decision to withdraw from Afghanistan, saying Monday that it was time to end U.S. military involvement in the country.
"I stand squarely behind my decision," Biden said. "After 20 years, I've learned the hard way that there was never a good time to withdraw U.S. forces. That's why we're still there."
Biden said he was faced with a choice to follow through with the drawdown or escalate the conflict.
When he was in office, former President Donald Trump negotiated a deal with the Taliban to withdraw U.S. military personnel by May 1. Biden in April set a timeline for Sept. 11.
Biden said Monday that U.S. officials had planned for every contingency, but "the truth is this did unfold more quickly than we had anticipated."
Biden has faced criticism for the chaos in Afghanistan and the events unfolding there.