Donald Trump Jr. has hired a lawyer to represent him in Russia-related investigations, his office and the lawyer said on Monday, as Republicans voiced concern about a meeting between the U.S. president's son and a Russian.

Trump Jr. hired New York lawyer Alan Futerfas, who specializes in criminal defence and whose clients have included reputed organised crime figures, computer hackers and white-collar criminals.

Futerfas would not say when he was retained or whether he played any part in the statements Trump Jr. made over the weekend about his June 2016 meeting at Trump Tower in New York with a Russian lawyer, Natalia Veselnitskaya, during the presidential election campaign.

Trump Jr. said he agreed to meet Veselnitskaya, described by the New York Times as having links to the Kremlin, after being promised damaging information about Democratic presidential candidate Hillary Clinton.

Trump's son-in-law, Jared Kushner, and Trump's then-campaign chairman, Paul Manafort, also attended, the Times said. It called the encounter the first confirmed private meeting of members of Donald Trump's inner circle with a Russian national during the campaign.

Allegations of the Trump campaign's ties to Russia have cast a shadow over the Republican president's first months in office and sparked investigations by congressional committees and a federal special counsel, Robert Mueller, into whether Russia interfered in the election and colluded with the Trump campaign.

Moscow denies interfering, and Trump, who became president on Jan. 20, says there was no collusion.

Futerfas told Reuters: “I look forward to assisting Donald Jr. and, quite frankly, there is nothing to all of the media buzz about the June 9th, 2016 meeting. That will be proven to be the case.”

A Republican member of the U.S. Senate Intelligence Committee, Susan Collins, called on Trump Jr. to testify before the panel, which is looking into accusations of Russian meddling in the election.

"Our intelligence committee needs to interview him and others who attended the meeting," Senator Susan Collins told reporters at the U.S. Capitol.

Senator Mark Warner, the top Democrat on the intelligence panel, said it "absolutely" wants to speak to Trump Jr. about the meeting. Warner said he and the Republican committee chairman, Richard Burr, will decide later whether to ask Trump Jr. to testify in public or in a classified setting.

'SOMEONE WE'LL WANT TO TALK TO'

"Rest assured, Donald Trump Jr. will be someone that we'll want to talk to," Warner told reporters outside his Senate office.

Burr would not say if the committee would talk to Trump Jr., but he told reporters the panel would look into the purpose of the meeting, who set it up and how the process unfolded before making a decision on who to interview.

"The committee will see everybody we think has value," Burr said.

White House spokeswoman Sarah Sanders said there was nothing inappropriate about the meeting with lawyer Veselnitskaya. "Don Junior took a very short meeting from which there was absolutely no follow-up," Sanders told reporters.

But Republican Senator Pat Toomey said it was not appropriate for presidential campaigns to solicit or accept negative information about a rival from a Russian.

"It encourages countries to come in and undermine our democratic process. And these countries have great capabilities to do that sort of thing," he told MSNBC.

In Moscow, Kremlin spokesman Dmitry Peskov told reporters on Monday that the Kremlin did not know the identity of the Russian lawyer.

"No, we don't know who it is and, certainly, we cannot track down all movements of all Russian lawyers both within Russia and abroad," Peskov said.

Public relations specialist Rob Goldstone said he arranged the meeting at the request of singer and businessman Emin Agalarov, a Moscow-based client of his. He told Trump Jr. the meeting was with a Russian lawyer who apparently claimed to have information regarding illegal campaign contributions to the Democratic National Committee.