President Donald Trump on Wednesday defended his son Donald Trump Jr, describing him as "innocent" and portraying him as the victim of a witch hunt.

His intervention followed revelations in emails that showed his eldest son welcomed Russian help against his father's rival in last year's presidential election.

"My son Donald did a good job last night," Mr Trump wrote on Twitter, referring to a Fox News television interview by his son on Tuesday night.

"He was open, transparent and innocent. This is the greatest Witch Hunt in political history. Sad!"

But it came as the New York Times reported that Mr Trump Jr's initial comments on the story, published by the paper on Sunday, were drafted on board Air Force One and were signed off by the president.

That statement was vague and inconclusive, and did not draw a line under the story. Instead, it led to days of further damaging revelations.