Trump 'very surprised' by son's subpoena in Russia probe

US President Donald Trump rushed to his son's defence Thursday, saying he was "very surprised" by a Senate panel's order that Donald Trump Jr testify as part of its investigation into Russia election interference.

"I was very surprised," Trump told reporters about the Senate Intelligence Committee's subpoena of his eldest of five children.

It was the first known legal summons issued by Congress to a member of the president's family to force testimony in the ongoing investigation.

The order comes after Special Counsel Robert Mueller decided there was insufficient evidence to accuse Trump's 2016 campaign of criminal conspiracy to collude with the Russians.

"My son is a very good person, he works very hard. The last thing he needs is Washington DC. I think he'd rather not ever be involved," Trump said.

Trump Jr has already testified at length to the committee, which is run by Republicans, and was "totally exonerated by Mueller," Trump said.

"For my son, after being exonerated, to now get a subpoena to go again, to speak again after close to 20 hours after telling everybody that would listen about a nothing meeting? Yeah, I'm pretty surprised."