BRUSSELS, BELGIUM (REUTERS) - James Mattis, the face the Trump administration's Pentagon, gave his Nato allies a strong warning: Honour military spending pledges or the United States may temper its support for the alliance.

This was said during a closed-door session at Nato's headquarters in Brussels, his first visit as secretary of defence.

"America cannot care more for your children's security than you do," Mattis told his counterparts, according to prepared remarks given to media.

He also accused some nations of ignoring threats, including from Russia.

Only a handful of Nato's European members spend its recommended two per cent of GDP on defence, a shortfall President Trump campaigned on, once calling the alliance "obsolete".

Since then, he has cooled off, saying he backs Nato - comments Mattis echoed in public on Wednesday (Feb 15).

"The alliance remains a fundamental bedrock for the United States and for all the transatlantic community. As President Trump has stated, he had strong support for Nato."

Now European members say they have increased spending by some $10 billion last year.

But the Nato secretary general says more is needed:"We still have a long way to go. It's not enough. We don't expect allies to meet the two per cent target immediately, but we expect all allies to stop the cuts and start to increase defence spending."

Trump isn't the only president to have called for spending increases. Predecessor Barack Obama often said the United States bore an undue burden.

But Trump has been far more assertive - and far more unsettling for Nato.