Washington's allies are waiting nervously for clarity from US President Donald Trump's administration after the volatile leader demanded that NATO more than double its spending target and vowed to end the war in Ukraine.

Hegseth's two days of talks in Brussels with his counterparts from NATO and Ukraine are part of a flurry of visits to Europe this week by top US officials.

Those will culminate with Vice President JD Vance meeting Ukrainian President Volodymyr Zelensky at a security conference in Munich on Friday.

"Arrived at NATO HQ. Our commitment is clear: NATO must be a stronger, more lethal force -- not a diplomatic club," Hegseth wrote on X. "Time for allies to meet the moment."

The Pentagon chief on Wednesday will sit down with an international coalition of Ukraine's backers before huddling with the 31 other defence ministers from NATO on Thursday.

Trump's return to the White House has put nerves on edge as he pushes his "America First" agenda and has generated uncertainty over US security commitments in Europe.

He has already rocked allies by announcing tariffs, and in Denmark's case insisting he wants to take over Greenland.

On both Ukraine and their own defence, Hegseth looks set to deliver the message that Washington expects European nations to do more.

"The European continent deserves to be free from any aggression, but it ought to be those in the neighbourhood investing the most in that individual and collective defence," Hegseth said on Tuesday as he kicked off his European trip at a US military base in Germany.

His combative boss has long accused allies of underpaying and says NATO should ratchet up its defence spending target from the current two percent of GDP to five percent.

That goal appears well out of reach for most -- but NATO chief Mark Rutte said he expects the goal to be raised to "north of three percent".

"In 2024, NATO Allies in Europe and Canada invested $485 billion in defence, a nearly 20 percent increase compared with 2023," Rutte said.

"But we need to do a lot more so we have what we need to deter and defend. And so that there is more equitable burden sharing."

Diplomats say they hope Hegseth will lay out the Trump administration's demands, firing a starting pistol on negotiations for setting a new target at a June summit in the Netherlands.

- 'Equalise' Ukraine aid -

The other core question is Trump's approach to Ukraine as Kyiv's forces struggle to hold back Russia after nearly three years of all-out war.

Trump has pledged to strike a quick deal to end the conflict, but so far Washington has given few indications about any plan.

Diplomats say they are not expecting Hegseth to lay out any details of how this will be achieved.

Instead, the Pentagon chief is expected to call on European allies to shoulder more of the burden on arming Ukraine.

"The Europeans have to own this conflict going forward," Trump's national security advisor Mike Waltz told US media.

Washington gave Kyiv tens of billions of dollars in military assistance under former president Joe Biden.

But Trump's administration has yet to provide details on new support, despite several billion dollars in remaining budget authorisation carrying over from his predecessor's time in office.

Rutte said that NATO allies had exceeded a pledge to give 40 billion euros of military aid to Ukraine last year -- with more than half coming from Europe and Canada.

"It also takes a big step in the direction of what President Trump has called for. I agree with him that we must equalise security assistance to Ukraine," Rutte said.

The meeting of Ukraine's supporters will for the first time be chaired by Britain and not the United States.

Several countries -- such as Britain -- are expected to use the occasion to make announcements of fresh military support for Ukraine.