Former President Donald J. Trump said on Saturday that, while president, he told the leaders of NATO countries that he would “encourage” Russia “to do whatever the hell they want” to countries that had not paid the money they owed to the military alliance.

Mr. Trump did not make clear whether he ever intended to follow through on such a threat or what that would mean for the alliance, but his comment at a campaign event in South Carolina — a variation of one he has made before to highlight his negotiation skills — is likely to cause concern among NATO member states, which are already very nervous about the prospect of a Trump return.

Mr. Trump’s suggestion that he would encourage Russian aggression against allies of the United States — for any reason — comes as Republicans in Congress have pushed back against more aid for Ukraine in its war against Russia, and as European officials have expressed concerns over possible Russian aggression on NATO’s Eastern side.

Russia’s president, Vladimir Putin, dismissed those warnings as “threat mongering” in an interview with Tucker Carlson, the former Fox News host, that aired on Thursday. “We have no interest in Poland, Latvia or anywhere else,” Mr. Putin said.

But he has also called on the United States to “make an agreement” to end the war in Ukraine by ceding Ukrainian territory to Russia, comments that were seen by some as an appeal to American conservatives to block further involvement in the war.

Some European officials and foreign policy experts have said they are concerned that Russia could invade a NATO nation after its war with Ukraine concludes, fears that they say are heightened by the possibility of Mr. Trump returning to the presidency.

Mr. Trump has previously expressed his belief that support for NATO is overly burdensome on the United States, saying the alliance drains its financial and military resources. His campaign website says that the country must re-evaluate the organization’s purpose.

He has in the past recalled privately telling NATO members that the United States would not defend them from Russian attacks if they were in arrears. Last year, he claimed during a campaign speech that “hundreds of billions of dollars came flowing in” to NATO after he made that threat.

On Saturday, he again brought up that anecdote, saying that he told European leaders they had to “pay up.”

Then, he said, the president of “a big country stood up and said, ‘Well, sir, if we don’t pay and we’re attacked by Russia, will you protect us?’”

Mr. Trump said he asked the other president if the country was “delinquent” in its payments. The leader responded, “Yes. Let’s say that happened,” Mr. Trump said.

“No, I would not protect you,” Mr. Trump recalled responding. “In fact, I would encourage them to do whatever the hell they want. You’ve got to pay. You got to pay your bills.”