US President Donald Trump said that it was "very unfair" for the Senate Intelligence Committee to subpoena his eldest son, Donald Trump Jr, to testify for a second time about alleged Russian interference in the 2016 presidential election.

"It's really a tough situation because my son spent I guess over 20 hours testifying about something that (special counsel Robert) Mueller said was 100 per cent OK," Xinhua quoted Trump as saying to reporters on Tuesday.

"And now they want him to testify again. I don't know why. I have no idea why. But it seems very unfair to me," he said.

The US President spoke as a news report emerged on Tuesday stating that Trump Jr backed out of two scheduled interviews with the Republican-controlled Senate panel before it issued a subpoena forcing him to appear.

Trump said last week that he was "very surprised" to learn of the subpoena and defended his son as a "good person" who "works very hard" and would be distracted by having to testify again.

Senators on the committee reportedly want to follow up on their 2017 interview of Trump Jr, who ran with others, regarding a campaign meeting in New York's Trump Tower in June 2016 with a Russian lawyer.

Russia has repeatedly denied any meddling in the US elections.

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