NATO Secretary General Jens Stoltenberg on Monday said "all options remain open" on Afghanistan, as allies wait for Washington to decide on a looming withdrawal deadline.

US Secretary of State Antony Blinken is heading to Brussels for two-days of meetings with NATO foreign ministers, looking to shore up ties on his first official trip to Europe.

High on the agenda is the future of the alliance's 9,600-strong mission in Afghanistan after former US leader Donald Trump struck a deal with the Taliban to withdraw troops by May 1.

"There are no easy choices and for now, all options remain open,” Stoltenberg said at a press conference.

"The security situation is difficult and we will take all the necessary measures to keep our troops safe.”

Current US President Joe Biden is reviewing the accord and said last week it would be "tough” for Washington to meet that date.

That prompted outrage from the Taliban, who warned that the US would be "responsible for the consequences”.

NATO allies have said they are willing to stay in Afghanistan longer, if Washington decides to remain as well.

There are some 2,500 US troops in the country and American support is vital to keep the NATO mission going.

Stoltenberg said the allies would "consult closely” with Blinken at the upcoming meeting.

Under the deal with the US, the Taliban vowed to engage in peace talks with the Afghan government, but the talks have made almost no progress and fighting has worsened — particularly in rural areas.

Major urban centres are also in the grip of a bloody terror campaign in the form of attacks targeting politicians, civil servants, academics, rights activists and journalists.

The US is scrambling to inject fresh impetus into a peace process that has dragged on in Doha since September, with feuding parties unable even to agree on an agenda.

Defence Secretary Lloyd Austin made an unannounced visit to Afghanistan on Sunday for talks with President Ashraf Ghani.

Last week Moscow hosted a gathering of stakeholders in a bid to break the deadlock, but even that ended without any concrete proposals.

An even broader conference is now scheduled to be hosted by Turkey next month.

The US and its allies are desperate to avoid seeing Afghanistan slip back into being a haven for terror groups, two decades after they intervened in the wake of the September 11, 2001 attacks in the United States.