NATO members pledged last year to ramp up defence-related spending to five percent of GDP but, despite increased efforts, many states say they may not be able to reach that target.

"For too long, polite pleas from our European allies to spend more on their own defence fell on deaf ears," the Pentagon chief said at a defence summit in Singapore.

"They are finally playing catch-up," Hegseth said in a speech at the annual Shangri-La Dialogue conference.

"Allies who refuse to step up and carry their own weight for our collective defence will face a clear shift in how we do business."

Secretary of State Marco Rubio said this month NATO was facing US troop cuts in Europe as Washington focused on other threats and European nations ramped up their defences.

In Asia, Hegseth reiterated that the region's security had "rested disproportionately on American military power, while many of our allies and partners allowed their own defence capabilities to atrophy".

Many countries in the Asia-Pacific region were indeed stepping up, Hegseth said, using South Korea as a particular example.

"South Korea has invested consistently in its own defence, because it does not have the luxury of treating war like an academic exercise.

"They live on the front lines, and so they build real combat power."

This "reflects simply a clear-eyed understanding of the threat environment", he said.

Hegseth also praised the spending policies of other countries including Australia, the Philippines and Japan.

"You don't have a strong alliance unless everyone has skin in the game. No freeloading," Hegseth said.

Hegseth agreed when asked by a New Zealand delegate whether he considered the Pacific island nation's plan to increase defence spending from one to two percent to be "freeloading".

"If I'm being honest, two percent is not enough, and so two percent is freeloading.

"I don't have anything against New Zealand, (but) I want partners to step up," Hegseth said.

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21ST CENTURY FOX