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".
Addressing crowds during a rally in South Carolina on Saturday, Mr Trump said he had made his comments about Russia during a previous meeting of leaders of Nato countries.
The former president recalled that the leader of a "big country" had presented a hypothetical situation in which he was not meeting his financial obligations within Nato and had come under attack from Moscow.
He said the leader had asked if the US would come to his country's aid in that scenario, which prompted him to issue a rebuke.