Pentagon press secretary John Kirby said "the threat remains high, and it remains real" outside Karzai International Airport airport in Kabul as the U.S. military faces its final hours ahead of President Biden's Aug. 31 deadline for a full military withdrawal. He said threats were "active and in many cases still specific."

"Non-combat evacuation operations are dangerous, period," Kirby said. "The end of them, particularly one in that, in an environment that we can't consider, clearly cannot consider permissive, are particularly dangerous."

"We are in a particularly dangerous time now," he added. "Not that it hasn't always been dangerous, but it is particularly dangerous now."

Kirby repeatedly declined to provide specific details about the withdrawal's final hours but did say the military had the ability to evacuate people "until the very end."

He said "yes" he was sure all troops would be out by the deadline but declined to say exactly what time the deadline would be -- whether it was Kabul time, or otherwise.

He also wouldn't say whether the U.S. planned to leave behind the U.S. military's anti-projectile C-RAM defense system.

"Commanders are in-flowing and out-flowing those requirements needed to complete the mission," he said.

As night fell in Kabul, and Pentagon officials declined to comment on whether gates to the airport are still open or under Taliban control, the U.S. officially entered its final hours to fully withdraw forces from Afghanistan after 20 years in the country.

President Biden is not scheduled to speak on the penultimate day of a military presence there under a deadline which he set, with no public events on his schedule, but is meeting in the Situation Room with his national security team on Monday morning.

Secretary of State Antony Blinken is scheduled to speak in the afternoon after hosting a summit with allies on Afghanistan. The summit is notably set to include both Turkey and Qatar, nations in talks with the Taliban about taking over the airport in Kabul, but absent will be China and Russia, which have both flirted with recognizing the militant group.

There are still up to 250 U.S. citizens in Afghanistan trying to evacuate, a State Department spokesperson told ABC News in a statement late Sunday, and thousands of Afghan allies desperate to get out. The Pentagon said on Monday "there is still time" for American evacuations.

The Biden administration has gotten nearly 100 countries and counting to sign onto another joint statement, saying they "will continue issuing travel documentation to designated Afghans, and we have the clear expectation of and commitment from the Taliban that they can travel to our respective countries."

But without a U.S. presence, the fate of the airport, and of the hopes of those still wanting to leave, is unclear. As forces retreat from the country, many of those remaining likely in the middle of the night, officials have warned the retrograde process of removing troops and equipment from a combat zone is the most dangerous part of a mission.

The final hours of the U.S. military’s presence in Afghanistan come after the bodies of the 13 service members killed in last Thursday’s attack arrived at Dover Air Force Base on Sunday, where Biden met with families, one of which has criticized him in the media for how the withdrawal has been handled.

With hours until Biden's deadline for a full military withdrawal from Afghanistan, Maj. Gen. Hank Taylor acknowledged at a briefing at the Pentagon on Monday reports of civilian casualties following the U.S. conducting an "over the horizon" airstrike Sunday that hit a vehicle "known to be an imminent ISIS-K threat."

"This self-defense strike successfully hit the target near Kabul airport," Taylor said. "Significant secondary explosions from the targeted vehicle indicated the presence of a substantial amount of explosive material. We are aware of reports of civilian casualties and we take these reports very seriously, and we are continuing to assess the situation."

Over the weekend, the U.S. struck two ISIS-K targets in Afghanistan, including a person believed to be planning future attacks. An Afghan health official reported that in one of those strikes, at least six Afghan civilians were also killed, including four children.

"Make no mistake, no military on the face of the Earth works harder to avoid civilian casualties than the United States military, and nobody wants to see innocent life taken," said Pentagon press secretary John Kirby. We take it very, very seriously, and when we know that we have caused innocent life to be lost in the conduct of our operations. we're transparent about it."

"Nobody wants to see that happen, but you know what else we didn't want to see happen? We didn't want to see happen what we believe to be a very real, a very specific and a very imminent threat to the Hamid Karzai International Airport and to our troops operating at that airport as well as civilians around it and in it," Kirby added.

As forces complete the retrograde process, which officials have said is the most dangerous part of a combat mission, Kirby warned "the threat stream is still real."

He reiterated that as many as five rockets were fired at the airport in Kabul late Sunday but said military forces "successfully employed our force protection measures to thwart that attack." ISIS-K has since claimed responsibility for the attack and claimed six rockets were fired.

Pentagon officials said one rocket was intercepted by the U.S. military's anti-projectile C-RAM system, one hit the airport but "landed with no effect to the mission, or any danger to our personnel and "three landed off the airfield with no effect."

ISIS-K has claimed responsibility for the attack that targeted Hamid Karzai International Airport in Kabul with six Katyusha rockets on Sunday, according to a communique issued by ISIS-K on Monday and translated by SITE Intelligence Group.

ISIS-K claimed the rocket landed with "direct hits."

The message comes just before nightfall in Kabul and one day before Biden’s Aug. 31 military withdrawal deadline from Afghanistan. 

A U.S. official confirmed to ABC News late Sunday that it appeared that as many as five rockets were fired toward the airport in Kabul. There were no signs of casualties at the time, and White House press secretary Jen Psaki issued a statement early Monday stressing that U.S. operations would continue uninterrupted.

ISIS-K also claimed responsibility for last week’s suicide bomber attack outside the airport which left 13 U.S. service members and 169 Afghans dead.

Over the weekend, the U.S. struck two ISIS-K targets in Afghanistan, including a person believed to be planning future attacks. An Afghan health official reported that in one of those strikes, six Afghan civilians were also killed, including four children.

-ABC News' Cindy Smith