The warning came ahead of a meeting of NATO defence ministers Thursday that will seek to forge a deal on hiking military spending for a summit later this month.

US President Donald Trump has called on Washington's allies to commit to spending five percent of their GDP on defence.

NATO chief Mark Rutte looks on track to secure a compromise deal agreement at the upcoming summit in the Hague for 3.5 percent of GDP on core military spending, and 1.5 percent on broader security-related areas such as infrastructure.

"The urgency of this moment is undeniable as the Russia-Ukraine conflict grinds on, Moscow is already preparing for its next move," US ambassador Matthew Whitaker told journalists.

"We are already seeing the Kremlin aims to rebuild its military. NATO allies must outpace Russia. We have no other choice. Let me be clear, the time is now."

Whitaker said "the United States expects every ally to step up with concrete plans, budgets, timelines, deliverables, to meet the five percent target".

"This is not going to be just a pledge. This is going to be a commitment. Every ally must commit to investing at least five percent of GDP in defence and security, starting now again, this is not a suggestion," he said.

The US envoy said that Washington remained committed to NATO's Article Five mutual defence clause -- but expected allies to step up their spending.

"We will defend every inch of allied territory, and we will do it from a position of unmatched strength," Whitaker said.

NATO ministers will sign off at their meeting in Brussels on new capability targets for the weaponry needed to face the threat from Russia.

"We are going to take a huge leap forward," Rutte said.

"These targets set out what forces and concrete capabilities the allies need."

Rutte said he was "absolutely, positively convinced" that NATO countries would agree to a new spending deal in The Hague.

The NATO chief insisted that the United States "have made totally clear their commitment to Article Five".

He similarly insisted that Washington remained committed to backing Ukraine despite defence secretary Pete Hegseth skipping a meeting of Kyiv's backers in Brussels on Wednesday.