Kirby said the evacuation operation at the airport, currently controlled by thousands of US troops, is on high alert after the Islamic State-Khorasan launched five rockets at the operation earlier Monday.

The group last week carried out a suicide bombing at one of the airport's gates killing more than 100 people, including 13 US troops.

And over the weekend a US drone-launched rocket destroyed a car that the Pentagon said was laden with explosives that the Afghan arm of the Islamic State group planned to use to target Americans.

"We're in a particularly dangerous time right now," Kirby told reporters. "The threat stream is still real, it's still active, and in many cases it's still specific."

US troops, whose numbers grew to 5,800 after the evacuation operation began on August 14, were already departing ahead of the August 31 deadline set by President Joe Biden to fully exit the country, now in the hands of the Taliban that US forces fought for two decades.

Kirby would not provide details of how the last hours of the pullout would unfold.

But he said officials had engaged the Taliban in extensive discussions to ensure it proceeds safely.

"We have been in communication with the Taliban about these final days, so that we can make sure there is no miscalculation, no misunderstanding," he said.

Major General Hank Taylor said more than 122,000 people have been evacuated from Kabul since July, including 5,400 Americans.

He said it will continue to be possible to evacuate US citizens still in Afghanistan to the last moment.

"The mission of the evacuation operation was to help as many people as possible leave Afghanistan," he said.

Taylor said the US is investigating widespread reports that a number of civilians, possibly nine members of the same family including several children, were also killed in Sunday's drone strike.

"We are aware, reports of civilian casualties and we take these reports very seriously, and we are continuing to assess the situation," he said.

Of the five rockets fired at the airport on Monday, Taylor said, three missed the target, one was shot down by US defense systems, and a fifth landed on the airport compound causing little damage and no injuries.

The Islamic State claimed responsibility for that attack.