Trump described Robert Mueller's testimony to Congress on the Russia probe as an "embarrassment" to US.

President Donald Trump on Wednesday described Robert Mueller's testimony to Congress on the Russia probe as an "embarrassment" to the nation, mocking his Democratic rivals for asking the former special counsel to appear on Capitol Hill.

Trump had said he would not watch the day's proceedings, but after the first of two congressional hearings in the House Judiciary Committee, he made his feelings known.

"I would like to thank the Democrats for holding this morning's hearing," Trump tweeted.

"Now, after 3 hours, Robert Mueller has to subject himself to #ShiftySchiff - an Embarrassment to our Country," he added, referring to the Democratic chairman of the House Intelligence Committee, Adam Schiff.

Grilled by lawmakers on his two-year investigation into Russian meddling in the 2016 presidential election, Mueller said he did not exonerate Trump, but once more refused to tell lawmakers if the president committed a crime.

Just before Trump's tweet, his spokeswoman Stephanie Grisham weighed in.

"The last three hours have been an epic embarrassment for the Democrats," she said, midway through the marathon day of hearings for the former special counsel.

"Expect more of the same in the second half."