By Sabine Siebold and Andrew Gray

MONS, Belgium (Reuters) -U.S. Air Force General Alexus Grynkewich took over on Friday as NATO's top military commander, sending a message of resolve and reassurance to allies concerned about potential cuts to U.S. forces in Europe by the Trump administration.

The U.S. has filled the post of Supreme Allied Commander Europe since its creation after World War Two but U.S. officials discussed ending that tradition as President Donald Trump pushes Europe to take more responsibility for its defence.

That possibility rattled European allies - who rely heavily on the 80,000 U.S. troops in Europe to protect the continent and would expect many more to join them if Europe came under attack - and alarmed senior members of the U.S. Congress.

Ultimately, Washington nominated Grynkewich as successor to Army General Christopher Cavoli, who played a key role in advising Ukraine's military after Russia's 2022 invasion and oversaw a sweeping overhaul of NATO's defence plans.

But U.S. allies remain concerned that an ongoing review of U.S. force posture worldwide could lead to troop cuts in Europe, just when they see Russia as a greater threat than at any time since the end of the Cold War.   

"These are consequential times," Grynkewich said at the handover ceremony, which took place in bright sunshine at NATO military headquarters outside the Belgian city of Mons, attended by military officers and officials from across the alliance.

"The threats we face are only becoming more intertwined, but they are no match for the unity, the resolve and the shared purpose that I see gathered on this field today," he declared.

"We stand ready in defence of our nations, and we're only going to get better," he said.

UKRAINE 'CAN WIN'

Speaking to reporters after the ceremony, Grynkewich - also known from his time as a fighter pilot by his call sign "Grynch" - said it was too early to say what the U.S. force posture review would conclude.

European leaders say they are ready to step up on security, having pledged big increases in military spending at a NATO summit last week, but want any transition to be gradual and orderly to avoid gaps in Europe's defences.

In his confirmation hearing in the U.S. Senate last week, Grynkewich said he thought Ukraine "can win" its fight against Russia's invasion and spoke in favour of continued support for Kyiv.

After the handover ceremony in Mons, he declined to comment on the recent U.S. decision to halt the delivery of some critical weapons and ammunition to Ukraine.

"It's a conversation that's ongoing," he told reporters. "I think we'll see more play out in the next week or two."

Besides his role at NATO, Grynkewich has also taken over as the head of U.S. European Command, based in Stuttgart in southern Germany, which oversees all U.S. forces in Europe.

(Reporting by Sabine Siebold and Andrew Gray; Editing by Hugh Lawson)