President Donald Trump says he doesn't "remember much" about a March 2016 meeting with his campaign foreign policy adviser that is now front-and-centre in special counsel Robert Mueller's Russia probe.

Speaking to reporters outside the White House before departing Friday for a lengthy trip to Asia, Trump said: "It was a very unimportant meeting. It took place a long time ago. I don't remember much about it."

Trump's former campaign adviser, George Papadopoulos, has pleaded guilty to lying to the FBI about his contacts with Russian officials. Papadopoulos later told agents he used that gathering to offer to arrange a meeting between Trump and Russian President Vladimir Putin.

Trump and the White House have branded Papadopoulos a campaign "volunteer" and a liar whose claims shouldn't be taken seriously.

The Trump campaign released a photo on Instagram dated March 31, of a meeting that included Trump and future attorney general Jeff Sessions at opposite ends of the table, with Papadopoulos also present.

According to court documents released this week, Papadopoulos brought up the possibility of arranging a meeting between Trump with Russian President Vladimir Putin.

'The saddest thing'

Trump has expressed frustration with the special counsel investigation headed by former FBI director Mueller, which can bring charges for any federal crimes it uncovers, but is primarily focused on whether Russians meddled into the 2016 election and whether Americans colluded with foreign parties.

He has previously publicly criticized Attorney General Jeff Sessions for recusing himself from any investigations on the Russia subject, which helped pave the way for Mueller's appointment.

Democrats, and some Republicans, fear Trump will seek to end Mueller's probe. The president previously fired FBI Director James Comey, saying to NBC News that "this Russia thing" helped motivate that decision.

In an interview Thursday with conservative radio host Larry O'Connor, Trump said: "The saddest thing is that because I'm the president of the United States, I am not supposed to be involved with the Justice Department," Trump said. "I am not supposed to be involved with the FBI. I'm not supposed to be doing the kinds of things I would love to be doing and I'm very frustrated by it."

Trump, as he has frequently, wondered aloud to O'Connor why the Justice Department wasn't "going after Hillary Clinton" with respect to the controversial dossier of Trump allegations that found its way into the hands of his rivals, both Republican and Democratic, during the campaign.

Clinton said in an appearance on Trevor Noah that there was a clear distinction in opposition research and working with foreign officials to gain an advantage in an election.

"Of course there is, I think most serious people understand that," said Clinton. "This was research started by a Republican donor during the Republican primary and then when Trump got the nomination for the Republican Party, the people doing it came to my campaign lawyer and said, you know, 'would you like us to continue it?' and he said, 'yes' … He knows what opposition research is."

Clinton added "the fact of the FBI investigation into the Trump campaign and Russia should have come out" during the campaign and not after the result.

Mueller's investigation has also ensnared former Trump campaign chair Paul Manafort and deputy campaign official Rick Gates. Manafort and Gates are under house arrest and accused of not registering as foreign agents on behalf of a pro-Russian Ukraine president, and then not disclosing millions in proceeds from those activities.

Mueller prosecutor Kyle Freeny wrote in a court filing made public Friday that the government will likely need 15 trial days to present its evidence supporting a 12-count indictment unsealed earlier this week alleging violations of federal money laundering, banking and foreign lobbying laws.