President Joe Biden has said the US is on "a pace" to complete its evacuation mission in Afghanistan by August 31 and doesn't plan to have troops in the country past that date, but the completion of the deadline depends on cooperation from the Taliban.

The US has currently approximately 5,800 troops at the Hamid Karzai International Airport in Kabul.

"We are currently on a pace to finish (the evacuation mission) by August 31. The sooner we can finish, the better. Each day of operations brings added risk to our troops," Biden told reporters at the White House on Tuesday.

"But the completion by August 31 depends on the Taliban continuing to cooperate and allow access to the airport for those who were transporting out and no disruptions to our operations," he said.

The Taliban -- which seized power in Afghanistan on August 15, two weeks before the US was set to complete its troop withdrawal after a costly two-decade war -- has warned that the US must end its evacuation mission on August 31.

Taliban spokesman Zabihullah Mujahid in a news conference in Kabul on Tuesday said the US must stick to its self-imposed deadline. "After that we won't let Afghans be taken out on evacuation flights," he said.

Any decision by the US to stay longer could lead to a war between them and the US troops who are executing the airlift at Kabul airport, the Taliban has said.

Biden said he has asked Pentagon and the State Department for contingency plans to adjust the August 31 timetable of leaving Afghanistan, "should that become necessary".

"I'm determined to ensure that we complete our mission...I'm also mindful of the increasing risks that I've been briefed on and the need to factor those risks in. They're real and significant challenges that we also have to take into consideration. The longer we stay, starting with the acute and growing risk of an attack by a terrorist group known as ISIS-K, an ISIS affiliate in Afghanistan, which is the sworn enemy of the Taliban as well," he said.

"Every day we're on the ground is another day we know ISIS-K is seeking to target the airport and attack both US and allied forces and innocent civilians," Biden said.

The Islamic State group's Afghanistan affiliate, ISIS-K, is known for staging suicide attacks on civilians.

Biden said though the Taliban are cooperating "so that we can get our people out. But it's a tenuous situation".

"We've already had some gunfighting break out," he said.

Earlier, White House Press Secretary Jen Psaki told reporters that the US is in direct contact not just with American citizens, but with SIV (Special Immigrant Visa) applicants as well as Afghans, whose departure the US is facilitating, about how and when to come to the airport.

Our expectation, which we have also conveyed to the Taliban, is that they should be able to get to the airport. It is also true that there are a number of Afghans who may not qualify for these programme. And we've seen, over the past nine days, a rush of people attempt to come to the airport. We certainly understand that, but that also creates security risk and one that we have great concern about, she said.

In a statement later in the evening, Pentagon Press Secretary John Kirby said that the retrograde of US non-combatant operation in Afghanistan has not been ordered and nor would it need to be ordered at this stage.

"The mission remains the same, and as you heard from the President today, it remains on the same timeline. We are focused on evacuating as many people as we can before the end of the month. The Secretary and military leaders are drawing up contingency plans should there be a need to reconsider this timeline. No such decision has been made," he asserted.

"As we have made consistently clear, commanders on the ground are empowered to make any adjustments they see fit, when they see fit. That includes changes to the footprint. To that end, we can confirm reports of the departure from Afghanistan of several hundred US troops," he said.

These troops represent a mix of headquarters staff, maintenance and other enabling functions that were scheduled to leave and whose mission at the airport was complete. Their departure represents prudent and efficient force management. It will have no impact on the mission at hand, Kirby said.

PTI