Donald Trump Jr. speaks ahead of his father President Donald Trump at a Make America Great Again rally in 2017. File picture: Mike Roemer/AP

New York - President Donald Trump was taken aback Thursday amid revelations that a senior Republican senator recently fired off a subpoena demanding the president's eldest son come in for a second round of questioning over his connections to Russia.

"I was very surprised," the president told reporters during an impromptu appearance at the White House, referencing the subpoena issued to Donald Trump Jr. by Senate Intelligence Committee Chairman Richard Burr.

Trump claimed Burr, a Republican from North Carolina, had declared "no collusion" following the release of special counsel Robert Mueller's report last month.

"My son was totally exonerated by Mueller, who frankly does not like Donald Trump, me, this Donald Trump," the president said, pointing at himself. "And frankly for my son after being exonerated to now get a subpoena to go again and speak again ... yeah, I'm pretty surprised."

Trump added, "My son is a very good person, works very hard. The last thing he needs is Washington, D.C."

Trump Jr. is the first known member of the president's immediate family to face a subpoena over Russia testimony.

A spokeswoman for Burr did not return a request for comment, and neither did an attorney for Trump Jr.

Axios first reported Wednesday that Burr had lobbed the subpoena because he and other members want to grill Trump Jr. over his 2017 testimony before the committee.

The committee, which continues to independently investigate Russia's interference in the 2016 election, expressed renewed interest in hauling Trump Jr. back in after former Trump fixer and personal lawyer Michael Cohen testified earlier this year that he had briefed the presidential son numerous times on plans during the campaign to develop a Trump-branded skyscraper in Moscow.

Cohen's claim contradicts Trump Jr.'s testimony before the committee that he was only "peripherally" aware of the Moscow project.

Additionally, Mueller's final report, citing testimony from Cohen, stated that Trump Jr. had informed his father ahead of time of a Trump Tower meeting in June 2016 attended by a Russian attorney who had promised political dirt on Hillary Clinton.

Trump Jr. told the Senate Intelligence Committee in his 2017 testimony that he had not told his father of the infamous meeting.

Lying to a congressional committee could be a crime, and Senate intelligence members are likely interested in finding out whether Trump Jr. deliberately misled them in 2017.

But President Trump - who has spent nearly two years lambasting Mueller as the ring leader of a "witch hunt" - pointed to the special counsel's report Thursday to defend his son.

"The Mueller report came out, that's the bible, and they said he did nothing wrong," the president said.

Despite the president's claims, Mueller revealed in his report that his team was strongly considering prosecuting Trump Jr. over possible campaign finance crimes stemming from the Trump Tower meeting.

However, the special counsel concluded he ultimately opted against doing so, saying it would be hard to prove that Trump Jr. was familiar enough with laws prohibiting foreign contributions of any kind to U.S. political campaigns.