A federal grand jury in Washington Friday approved the first indictments in the Russia investigation led by special counsel Robert Mueller, CNN reported.

The network cited "sources briefed on the matter," noting that the charges remained sealed by a federal judge's order.

The nature of the charges remained unclear.

"Plans were prepared Friday for anyone charged to be taken into custody as soon as Monday," CNN reported.

A Mueller spokesman declined to comment, but the network reported that top lawyers on the counsel's team—including longtime prosecutor Andrew Weissmann — were seen entering a federal courtroom in Washington where the grand jury meets.

Mueller was named by Deputy Attorney General Rod Rosenstein in May to oversee the agency's investigation into Russian meddling in last year's presidential election.

Rosenstein has oversight over the Russia investigation — and Mueller's team is required to inform him of any charges before pursing grand jury indictments, according to the report.

Mueller, who was appointed after President Donald Trump fired FBI Director James Comey, was authorized by Rosenstein to investigate "any matters that arose or may arise directly from the investigation."

Mueller also is a former FBI director, from 2001 to 2013.

President Trump has slammed the probe as a "witch hunt" and took to Twitter earlier Friday to attack it as "costly" and that it has proved "no collusion between Russia and Trump."

He also said that collusion had occurred between the Kremlin and Democrat Hillary Clinton.

It is now commonly agreed, after many months of COSTLY looking, that there was NO collusion between Russia and Trump. Was collusion with HC! — Donald J. Trump (@realDonaldTrump) October 27, 2017

Russia also has denied any role in the election.

Among the issues Mueller's team is investigating are the circumstances surrounding Trump's firing of Comey and whether the president was trying to derail the agency's Moscow probe.

In recent weeks, Mueller's team has interviewed several former members of Trump campaign or White House staff, including former Chief of Staff Reince Priebus, and former Communications Director Sean Spicer.

Priebus is said to have argued against Comey's termination, according to news reports, though he was not considered a focus of the investigation.

In addition, Mueller's team has also examined foreign lobbying efforts by former Trump campaign chairman Paul Manafort and former National Security Adviser Michael Flynn.

In July, the FBI seized documents from Manafort's home in Virginia in a pre-dawn raid armed with search warrant that had been obtained by Mueller's investigators.

Under Comey's leadership last year, his team obtained approval from the secret court established under the Foreign Intelligence Surveillance Act to wiretap Manafort's communications — as well as those of former Trump campaign adviser Carter Page.

Jason Maloni, Manafort's attorney, last month slammed the Mueller probe as "entirely politically motivated" and said that his client would no longer cooperate with the investigation.

Page told the Senate Intelligence Committee earlier this month that he would not testify in its Russia probe, even though he had asked House investigators months earlier for an open session to publicly present his case.

The Senate Judiciary Committee is also probing Moscow's role in last year's election.