Pentagon spokesman John Kirby said on Friday there could be an increase in the number of troops in Afghanistan in order to complete a safe withdrawal of all foreign forces by September 11.

Addressing a press conference Friday, Kirby said: "It is not out of the realm the possibility that for a short period of time, there will have to be some additional enabling capabilities added to Afghanistan, to help effect a safe, orderly, and deliberately planned drawdown of everybody by the president's deadline, early September."

Kirby did not however elaborate on the nature of the increase in US troops. "I can't speak today with exactly what that would look like, how many, when they would be going in. But as we transitioned out of Iraq, of course, it's logical to assume that you may need some logistics help, maybe some engineering help."

"You may have to add some force protection capabilities, temporarily, just to make sure that the drawdown goes in a safe, orderly, and effective way. Again, we'll know more as we get closer, but that would not be out of the realm of possibility," Kirby said.

Responding to a question, Kirby said that the Department of Defense is still working out what the future bilateral security relationship is going to be with Afghanistan.

"We're going to transition to a bilateral relationship, a military relationship that's more akin to the kinds of relationships we have with other countries," he said.

"It will not include a US military footprint on the ground in Afghanistan with the exception of what's going to be required to support the diplomatic mission there. All that's still being worked out. So, I can't speak with specificity today about what contract support the Afghan security forces are going to need going forward," he said.

The United States, he said, will maintain counterterrorism capabilities to continue to prevent Afghanistan from becoming a launching pad for terrorist attacks on the US.

"We have a vast range of capabilities available to us and we're not going to speak to the details of exactly how we're going to maintain those capabilities and utilise those capabilities," he said.

On Wednesday US President Joe Biden and NATO Secretary General Jens Stoltenberg announced that a full withdrawal of troops would start from May 1 and end by September 11 - the 20th anniversary of the 9/11 attack on the United States.

The Taliban did not however welcome the decision and said on Thursday the US was in breach of its agreement with the group, signed in February last year that called for full withdrawal by May 1.

Responding to the announcement, Taliban spokesperson Zabihullah Mujahid threatened retaliation and said on Twitter: "If the agreement is breached and foreign forces fail to exit our country on the specified date, problems will certainly be compounded and those (who) failed to comply with the agreement will be held liable."

Kirby meanwhile acknowledged the danger and responded with a warning.

"We've seen their threats, and it would be imprudent for us not to take those threats seriously," Kirby said.

"It would also be imprudent for the Taliban to not take seriously what the president and what (Defense) Secretary (Lloyd) Austin both made clear, that any attack on our drawdown, on our forces or our allies and partners as they draw down, will be met very forcefully."

Kirby also insisted that even though American forces will be leaving, "it doesn't mean that we're walking away from our Afghan partners."

U.S. officials have promised to continue paying the salaries of Afghan security force members and to financially support the Afghan Air Force.

Kirby said that going forward, the U.S. will continue to support the Afghan military.