America's top diplomat adopted a positive tone at the start of a two-day meeting of alliance foreign ministers as he looks to rebuild ties strained by former US leader Donald Trump.

But high on the agenda for NATO is the future of the alliance's 9,600-strong mission in Afghanistan after Trump struck a deal with the Taliban to withdraw troops by May 1.

Allies are waiting anxiously for new US leader Joe Biden to decide whether to stick to that date -- but Blinken said the US was still mulling it over.

"We have a review under way in the United States. I'm here today in part to share some of our initial thinking with our NATO allies," Blinken said in opening remarks with alliance chief Jens Stoltenberg.

"But maybe even more important, I'm here to listen and consult because that is what allies do."

Biden said last week that it would be "tough" for Washington to meet the looming deadline.

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 the US remains too, but they are keen for Washington to make up its mind.

Trump slashed US troop numbers to 2,500, the lowest level in two decades, but American support remains vital to keep the NATO mission going.

"Are we staying after May 1 or are we leaving," one European foreign minister asked, speaking to AFP on condition of anonymity. "We don't know what we are going to do and that is a problem."

Last year's deal saw the Taliban commit to peace efforts and cutting violence but the US has complained that talks have stalled and bloodshed has flared.

Washington is scrambling to inject fresh impetus into the peace process and Pentagon chief Lloyd Austin made an unannounced visit to Kabul Sunday.

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.

"Whatever the United States ends up doing will be informed by the thinking of our NATO allies, which I'll take back to me after these conversations," Blinken said.

"We are united with our NATO allies in seeking to bring a responsible end to this conflict, and to remove our troops from harm's way."

- 'Rebuild our partnerships' -

The visit by Blinken, which also includes meetings with top EU officials, aims to make good on Biden's assurances that the US wants to work closely with its allies.

"I've come to Brussels because the United States wants to rebuild our partnerships, first and foremost, with our NATO allies," Blinken said.

"We want to revitalise the alliance, to make sure it's as strong and effective against the threats of today, as it has been in the past."

Stoltenberg is currently hammering out proposals to reform the 72-year-old alliance, set to be pitched to leaders including Biden at a summit later this year.

NATO is looking to tackle growing threats from Russia and China and emerging security concerns such as cyberattacks and climate change.

But internal challenges remain for the 30-nation grouping, including bolstering defence budgets and squabbles with NATO member Turkey.

Trump harangued leading nations like Germany for failing to boost defence spending.

Despite the marked change in tone, the new US administration is expected to remain firm on pressing other members to do more to share NATO's financial and military burden.