The US Senate Judiciary Committee says that it has called President Donald Trump's eldest son, Donald Trump Jr., and former Trump campaign manager Paul Manafort to testify on July 26 at a hearing on alleged Russian meddling in the 2016 election.

Trump Jr and Manafort are expected to be questioned about allegations that Russia meddled in the 2016 US presidential election and possible collusion with the Trump campaign.

Another Trump relative, his son-in-law Jared Kushner, will be interviewed by US senators on Monday, Kushner's attorney said.

Senate aides declined to confirm his appearance.

"Working with and being responsive to the schedules of the committees, we have arranged Mr. Kushner's interview with the Senate for July 24," Kushner's attorney, Abbe Lowell, said in a statement. "He will continue to cooperate and appreciates the opportunity to assist in putting this matter to rest."

The Judiciary Committee announced Trump Jr. and Manafort's appearance in a statement on its website.

Senator Sheldon Whitehouse, a Democratic member of the Judiciary Committee, said the hearing would enable the panel to begin to get testimony under oath.

"There has been an enormous amount that has been said publicly but it's not under oath, which means that people are free to omit matters or lie with relative impunity," Whitehouse told CNN.