WASHINGTON (AFP) - President Donald Trump on Wednesday (March 20) launched a prolonged attack on the integrity of a huge probe into whether he or his election campaign colluded with Russia or obstructed justice - but did say that the final report should be released.

"If you want to let them see it, let them see it," Trump said of the report, which independent special prosecutor Robert Mueller is believed to be close to finishing.

Trump has been investigated for almost two years, following suspicions that he or his team had improper ties to Moscow.

Mueller will submit his findings to the Trump-appointed attorney general, who is not obligated to release the report. But pressure is intense for the contents to be made public.

In anticipation, Trump intensified his long-running campaign to question the legitimacy of Mueller, who is widely respected, and to undermine confidence in the report itself.

Trump referred to his 2016 election win as "one of the greatest... in the history of this country" and said by contrast that Mueller had no popular support.

Mueller is "a man out of the blue" and "writing a report that never got a vote," Trump told journalists at the White House.

Mueller, a former FBI director, was appointed in May 2017 with widespread bipartisan backing in Congress, four months into Trump's turbulent presidency.

Trump has repeatedly attacked the probe into his Russia ties as "a witch hunt" and is expected to raise the volume as he starts campaigning for reelection in 2020.

"My voters don't get it and I don't get it," he said of the Mueller probe.