"For me, there is absolute clarity that the United States is totally committed to NATO, totally committed to Article Five," Rutte told reporters ahead of a summit meeting of alliance leaders.

On his way to the summit, Trump refused to commit to NATO's Article Five clause, the basic agreement that says an attack on one member is an attack on all.

"Depends on your definition. There's numerous definitions of Article Five," Trump told journalists in comments sure to rattle America's European allies.

"I'm committed to being their friend," he said.

Rutte said that there was, however, an "expectation" that the Canadians and the Europeans would ramp up their spending, with the US long complaining that it paid too much to defend countries in Europe.

NATO allies, later on Wednesday, are to sign off a pledge to spend 3.5 percent of GDP on core defence spending -- just over the current US level -- plus another 1.5 percent on broader security-related areas such as cybersecurity and infrastructure.

"This is fair, that we spend the same as the US is spending," said Rutte.