NATO allies agreed Wednesday to start withdrawing their forces from Afghanistan by May 1 after the United States announced it would pull troops out by September, an alliance statement said.

"This drawdown will be orderly, coordinated, and deliberate," the statement said after talks involving defence and foreign ministers.

"We plan to have the withdrawal of all US and Resolute Support Mission forces completed within a few months."

NATO Secretary-General Jens Stoltenberg said the allies "went into Afghanistan together, we have adjusted our posture together, and we are united in leaving together.

"This is not an easy decision and it entails risks," he said.

"As I've said for many months, we face a dilemma. Because the alternative to leaving in an orderly fashion is to be prepared for a long-term, open-ended military commitment with potentially more NATO troops."

US President Joe Biden on Tuesday announced that Washington would withdraw its remaining 2,500 troops from Afghanistan by September 11 -- 20 years after the attacks in America that sparked its longest war.

The drawdown delays only by around five months a May 1 deadline agreed with the Taliban by former US President Donald Trump to withdraw troops, amid a growing consensus in Washington that little more can be achieved.

The 9,600-strong NATO training and support mission, which includes the US troops and depends heavily on Washington's military assets, includes personnel from 36 NATO members and partner countries.

The withdrawal comes despite peace efforts between the Taliban and the Afghan government stalling, with the insurgents saying they would boycott upcoming talks in Turkey.

"The future of Afghanistan, ultimately, is in the hands of the Afghan people where it belongs," US Secretary of State Antony Blinken said alongside Stoltenberg in Brussels.  "But our support, our engagement and our determination remain."