Democratic California Rep. Adam Schiff said Monday that he believes it is “highly unlikely” the U.S. military can finish evacuating Americans and Afghans from Afghanistan by the Aug. 31 deadline set by the Biden administration.

“I think it’s possible, but I think it’s very unlikely,” Schiff said, responding to a reporter’s question outside the Capitol after a classified briefing. “Given the number of Americans who still need to be evacuated, the number of SIVs [Special Immigrant Visas], the number of others who are members of the Afghan press, civil society leaders, women leaders, it’s hard for me to imagine all of that can be accomplished between now and the end of the month.”

He said that he was “encouraged” to see the number of evacuations increase daily, but added that the U.S. should maintain “a military presence as long as it’s necessary to get all U.S. persons out, and to meet” the country’s “moral and ethical obligation to our Afghan partners.”

Schiff expressed concern that terrorists such as ISIS or al-Qaida could target Hamid Karzai International Airport, describing the threat as “very real and very substantial.” (RELATED: ‘Al-Qaida, ISIS, The Haqqani Network’: Liz Cheney Warns Of Renewed Terrorist Threat)

“This would make a very attractive target for ISIS-K [Islamic State in Iraq and Syria-Khorasan] or for elements of al-Qaida, but most particularly ISIS-K. An opportunity to attack both U.S. forces as well as Afghan nationals, and indeed the Taliban as well, and just cause chaos that might prolong the U.S. stay in a way that ISIS-K believes might suit their interests.”

President Joe Biden and his cabinet members, including Secretary of Defense Lloyd Austin and national security adviser Jake Sullivan, have wavered on the end date of the military withdrawal. Sullivan told reporters during an Aug. 17 press conference that he would not “comment on hypotheticals” when asked if the U.S. would push back the withdrawal date to finish evacuations. One day later, Biden told ABC’s George Stephanopoulos that the U.S. would stay in Afghanistan “until we get them all out.”

Austin said Sunday that the U.S. would try its “very best to get everybody” out of Afghanistan, but that he had not yet recommended staying past Aug. 31 or made a proposal to Biden.

A spokesman for the Taliban, Dr. Suhail Shaheen, told Sky News on Monday that it would view an extension of the withdrawal date as a “red line” from the U.S.

“If the U.S. or U.K. were to seek additional time to continue evacuations – the answer is ‘no.’ Or there would be consequences. It will create mistrust between us. If they are intent on continuing the occupation it will provoke a reaction,” he said.

The White House reported early Tuesday morning that roughly 21,600 people were rescued in a 24-hour span from Aug. 23 to Aug. 24.