America's top diplomat adopted a positive tone at the start of a two-day meeting of NATO foreign ministers as he looks to rebuild ties strained by former US president 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 President 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 at the start of talks.

"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" if it fails to meet the deadline.

- 'Are we staying or leaving?' -

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.

"We don't want to run the risk of the Taliban returning to violence and trying to gain power by military means by withdrawing from Afghanistan too early," German Foreign Minister Heiko Maas said.

"That's why we hope that we can also talk concretely with the United States today about how the next few weeks will shape up."

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.

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 on 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.

"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."

- '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.

NATO chief Jens 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.

"It's no secret that we have differences with Turkey," Blinken said, pointing to tensions in the eastern Mediterranean and Ankara's purchase of a Russian missile defence system.

"It's also no secret that Turkey is a long standing and valued ally, and one that I believe we have a strong interest in keeping anchored to NATO."

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

Despite a 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.