Former US President Donald Trump said Thursday that he does not know "exactly" what to say after the Russian leader threw his support behind Vice President Kamala Harris in November’s presidential election.

"Putin came out today. He endorsed Kamala, and I didn't know what -- supposed to call him up and say, 'Thank you very much. I appreciate.' But, he endorsed Kamala.

"I have a feeling. I don't know exactly what to say about that. I don't know if I'm insulted or he did me a favor," Trump said at the New York Economic Club.

His remarks came after Vladimir Putin said Russia supports Harris, who replaced US President Joe Biden on the Democratic Party's presidential ticket after Biden dropped out if the race.

"I said that our favorite, if I may say so, was the incumbent President Mr. Biden. He was removed from the race, but he recommended that all his supporters support Ms. Harris. So, we will do the same, we will support her," Putin said at an economic forum in Russia.

The Russian president then joked that since Harris "laughs so expressively and contagiously," it means that "she is doing well" and she would not impose more sanctions on Russia.

Despite the caveat that it is the American people who decide, Putin’s remarks drew a swift rebuke from Washington.

"Putin ought to stop talking about our elections, period. He shouldn't be favoring anybody one way or another," John Kirby, spokesperson for the US National Security Council, told reporters.

Kirby added that Putin “should also stop interfering” in the election, one day after the US announced charges for alleged Russian attempts to sway the Nov. 5 polls.

Biden was set to represent the Democratic Party in the election, but he announced his withdrawal from the race on July 21 and endorsed Harris.

Trump, who has had friendlier relations with Putin than Biden or Harris, received the Republican Party’s presidential nod.