Trump suggested the United States might not protect NATO allies who aren't spending enough on defence from a potential Russian invasion.

"Quite frankly, I think this is exactly what Putin loves to hear," Dutch Defence Minister Kajsa Ollongren told Reuters on Monday. "Our strength is our unity. And if we're not united, it makes us weaker. And we know that that is what Putin is looking for."

Enshrined in Article 5 of NATO's founding treaty is the principle of collective defence - the idea that an attack on one member is considered an attack on all.

"It's not the first time that he says something along these lines. But at the same time, it's of course worrying," Ollongren said.

"...And I think especially now, with real threats also to NATO countries, it's very concerning that someone who is that important is saying these things."

Ollongren also noted that most NATO allies were close to or had reached the target budget spending on defence of a minimum of 2% of gross domestic product.

(Reporting by Anthony Deutsch; Editing by Nick Macfie)