Donald Trump's suggestion the US would not protect Nato allies failing to spend enough on defence "undermines all of our security", the Western military alliance's chief has said.
Jens Stoltenberg also suggested it put US and European troops at greater risk.
The Republican said he had told allies he would "encourage" Russia to attack any Nato member that failed to meet the alliance's target of 2% of their GDP.
Members of Nato commit to defend any nation in the bloc that gets attacked.
President Joe Biden called Mr Trump's comments "appalling and dangerous", suggesting his predecessor intended to give Russian President Vladimir Putin "a green light for more war and violence".
Nato chief says Donald Trump comments 'undermine all of our security'