Washington's top diplomat will attend a two-day meeting of NATO foreign ministers and hold talks with EU chiefs as he tries to mend relations that were strained under former US president Donald Trump.

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 for new US leader Joe Biden to make a decision on whether he is sticking to that date -- but no concrete announcement is expected from Blinken.

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

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

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 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.

"We went in together. We will adjust together as we have over the years. And when the time is right, we will leave together," US Assistant Secretary of State Philip Reeker said.

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 flared.

Washington is scrambling to inject fresh impetus into a peace process that has dragged, 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.

- 'Revitalising alliances' -

The visit by Blinken to Brussels aims to make good on Biden's assurances that the US hopes to strengthen the "transatlantic partnership" after the Trump years.

Washington says he will "deliver an important foreign policy speech focused on his commitment to rebuilding and revitalising alliances" in Belgium.

NATO is looking to paper over divisions at the 72-year-old alliance, but a raft of challenges remain, including bolstering defence budgets and squabbles with NATO member Turkey.

Trump harangued leading nations like Germany for failing to pull their weight as he pressured them to boost defence spending to two percent of gross domestic product.

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.

At the EU, Blinken is to discuss pressing challenges including Russia and China with European Commission President Ursula von der Leyen and foreign policy chief Josep Borrell.

Washington is keen to enlist Brussels in a united front of democracies against the rising power of Beijing.

The two sides -- along with Britain and Canada -- on Monday made a first step towards coordinated actions, with synchronised sanctions over China's crackdown on the Uighur minority.