"It is going to be hard to meet the May 1 deadline in terms of tactical reasons. It's hard to get those troops out," Biden said in his first press conference since taking office on January 20.

"We will leave, the question is when we leave," he said. "But we're not staying a long time."

Asked whether he envisions US soldiers still in Afghanistan in 2022, the president said: "I can't picture that being the case."

Under an agreement negotiated by the Donald Trump administration, the United States is supposed to pull out all its troops by May 1 in a deal that saw the Taliban agree to peace talks with the administration of Afghan President Ashraf Ghani.

Biden's acknowledgement comes one week after he said he was in the process of making a decision over the troops, and that a withdrawal on time "could happen, but it is tough."

Last week the Taliban warned there would be "consequences" if Washington did not stick to the agreed timetable -- further raising pressure on the fragile peace process.

On Thursday, Biden noted ongoing international discussions on Afghanistan, including Secretary of State Antony Blinken's meetings with NATO allies that have troops in Afghanistan.

"And if we leave, we're going to do so in a safe and orderly way," Biden said.

The Taliban have largely stuck to a promise not to attack US or other foreign troops since the agreement was struck in February last year, but they say the date to end America's longest war is inflexible.

The Taliban also vowed not to allow territory to be used by "terrorists" -- the original goal of the US invasion following the September 11, 2001 attacks.

But the supposed peace talks held in Qatar since September have made little progress.