NATO chief Jens Stoltenberg on Sunday warned against talk that "undermines security" after former US president Donald Trump threatened to encourage Russia to attack NATO members who were not fully paid up.

"Any suggestion that allies will not defend each other undermines all of our security, including that of the US," Stoltenberg said in a statement.

Trump threatened Saturday, in the event he is re-elected in the United States, that he would not defend NATO members who had not met their financial obligations, and going further would "encourage" Russia to attack them.

The former US president has regularly criticised allies of the North Atlantic Treaty Organization for underfunding the group.

European Council President Charles Michel earlier on Sunday also denounced his comments as "reckless".

"The Transatlantic Alliance has underpinned the security and the prosperity of Americans, Canadians and Europeans for 75 years," said Michel on X, formerly Twitter.

"Reckless statements on #NATO's security and (Article) 5 solidarity serve only Putin's interest" and "they do not bring more security or peace to the world", he added.

"On the contrary, they re-emphasise the need for the #EU to urgently further develop its strategic autonomy and invest in its defence," he continued.

Article 5 stipulates that if a NATO country is victim of an armed attack, each member of the Alliance must consider the act as an attack against the whole group and take measures to provide aid to the country under threat. Trump, likely the candidate to face Democrat President Joe Biden in November's election, made his statement about NATO after having put pressure on Republicans in Congress to block a bill to provide new aid to Ukraine.

The White House reacted sharply to Trump's remarks. "Encouraging invasions of our closest allies by murderous regimes is appalling and unhinged," White House spokesman Andrew Bates said Saturday night.

Stoltenberg on Sunday noted it also "puts American and European soldiers at increased risk".

"I expect that regardless of who wins the presidential election the US will remain a strong and committed NATO Ally," he added.