US President Donald Trump has ramped up his attacks on the Russia investigation by demanding it end "right now".
In a series of tweets, he said Attorney General Jeff Sessions must halt the inquiry into alleged election meddling.
Mr Trump also labelled special counsel Robert Mueller, who leads the probe, "totally conflicted".
His remarks are notable because he himself has previously acknowledged the US Department of Justice should be free from political interference.
According to US media, some Trump aides believe that Mr Mueller will report soon on the findings of his 14-month investigation as they relate to the president.
In a fusillade of tweets on Wednesday, the Republican president also lashed out at the case against his former campaign manager, Paul Manafort, whose trial is entering its second day.
Mr Manafort's trial is the first arising from Mr Mueller's investigation.
Mr Trump also said Mr Manafort was being treated worse than the Prohibition-era Chicago mobster Al Capone.
White House spokeswoman Sarah Sanders later said at a press briefing that the tweet is "not an order".
She described it as Mr Trump's "opinion", adding: "I think he's been crystal clear about how he feels about the investigation from the beginning."
Analysis by Jon Sopel, BBC North America Editor
There is nothing new about the president's frustration, agitation and anger at the special counsel investigation.
But this is the first time in overt terms that he has called on the attorney general to fire Robert Mueller. The timing is interesting too - his former campaign manager, Paul Manafort, has just gone on trial, accused of failing to declare the extent of his business connections with pro-Moscow Ukrainian politicians.
It also comes as the Mueller inquiry starts moving closer to the Trump family circle - in particular, the behaviour of his son, Donald Jr and son-in-law Jared Kushner over a meeting at Trump Tower during the campaign with a Russian lawyer who had close links to the Kremlin.
Technically the Attorney General, Jeff Sessions, can't fire Mueller as he has recused himself from the Russia investigation.
Ironically, one of the areas the Mueller investigation is looking at is whether the president has used Twitter to try to pressurise or intimidate his attorney general.
The president's tweet today might be seen as relevant.
Mr Manafort, 69, led Mr Trump's campaign for three months until August 2016.
He is charged with tax evasion and money-laundering stemming from his political consultancy work in Ukraine.
But Mr Manafort is not accused of aiding Russia's alleged attempts to sway the 2016 presidential election Mr Trump's way, a focus of the Mueller investigation.
The special counsel is also scrutinising whether there were any attempts by the president to obstruct justice.
The president wields significant constitutional authority over the Department of Justice since it belongs to the executive branch of US government.
But it has been a US civic tenet ever since the Nixon-era Watergate scandal that the agency should be free from political interference.
Mr Trump has previously acknowledged it is "the saddest thing" that he is not supposed to meddle with the law agency.
What's the reaction?
Utah's Republican Senator Orrin Hatch told journalists that many "wish the Mueller investigation would go away," but added, "that's not going to happen".
The third highest-ranking Senate Republican, John Thune of South Dakota, told reporters on Capitol Hill: "Most of us up here believe the process needs to play out, and it will."
Democratic congressman Adam Schiff called the president's words "an attempt to obstruct justice hiding in plain sight".
What's the latest in the Manafort trial?
On Wednesday, District Judge TS Ellis took issue with the term "oligarch" - which prosecutors used to describe the Ukrainian politicians for whom Mr Manafort worked - calling the word "pejorative".
"We're not going to have a case tried that he associated with despicable people and, therefore, he's despicable," the judge said. "That's not the American way."
On Tuesday, prosecutors called as their first witness Democratic strategist Tad Devine, who worked for presidential candidate Bernie Sanders during the 2016 election.
Mr Devine described how in 2010 they both assisted now-deposed Ukrainian leader Viktor Yanukovych during his presidential campaign.
The court also heard on Wednesday from an FBI agent who led the raid on Mr Manafort's Virginia home, an incident that has been heavily criticised by Mr Trump.
Matthew Mikuska testified that he knocked three times on the door before using a key to open it.
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