Paul Manafort. Chip Somodevilla/Getty Images

Talks between Robert Mueller, the special counsel leading the Russia investigation, and Paul Manafort, the former Trump campaign chairman, have broken down in recent days, ABC News reported Friday.

According to the terms of his plea agreement, Manafort is required to cooperate in any and all matters the government deems relevant.

The report came after President Donald Trump ousted Attorney General Jeff Sessions and appointed Matthew Whitaker, who has previously slammed Mueller and the Russia investigation, as the acting attorney general.

"This was one of the biggest fears about Whitaker in law-enforcement circles," a former senior Justice Department official who used to work with Mueller told Business Insider.

"His appointment not only endangers the status of the Russia investigation itself, but it could have an effect on the cooperation of those who have already pleaded guilty and others who Bob Mueller may be working to flip," this person said.

Talks between the special counsel Robert Mueller and Paul Manafort, the former chairman of President Donald Trump's campaign, have broken down in recent days, ABC News reported Friday.

Manafort struck a plea deal with Mueller's team in September, shortly after he was convicted on eight counts related to tax fraud, bank fraud, and failure to report foreign bank accounts. Before his second trial, Manafort pleaded guilty to conspiracy and obstruction and agreed to cooperate with Mueller.

Since then, Manafort has met with prosecutors nearly a dozen times, and though members of Mueller's team have been asking him about a wide range of topics, they're "not getting what they want," a source with knowledge of the discussions told ABC News.

Andrew Weissmann, a prosecutor working for Mueller, told US District Judge Amy Berman Jackson when Manafort's plea deal was announced that Manafort would cooperate "in any and all matters as to which the government deems the cooperation relevant," including "testifying fully, completely" before a grand jury.

Manafort's plea agreement also says that if he "has failed to cooperate fully" or "intentionally given false, misleading or incomplete information or testimony," he "will not be released from his pleas of guilty but the Government will be released from its obligations."

Read more: Robert Mueller has reportedly begun drafting his final report in the Russia probe

A source with direct knowledge of the matter told Business Insider that Manafort's plea deal with Mueller was still intact. But ABC News reported that the dispute between the two sides stemmed from suspicions that Manafort was not being entirely forthcoming about all the information he may know that's relevant to the Russia investigation.

The former Trump campaign chairman is a significant figure of interest in the inquiry, which is examining Russia's interference in the 2016 election, whether the Trump campaign colluded with Moscow, and whether Trump sought to obstruct justice after he learned of the investigation's existence last year.

As Manafort's cooperation stalls, Trump appoints a Mueller critic to oversee the Russia investigation

It's unclear why talks between Manafort and Mueller have apparently stalled. Manafort's team initially mounted an aggressive defense against Mueller, reportedly in hopes of catching the president's attention and prompting him to come to Manafort's aid. But Manafort shifted gears and struck a plea deal after his first trial resulted in a conviction.

Meanwhile, the landscape of the FBI's Russia investigation could change drastically in the coming months, as Trump ousted Attorney General Jeff Sessions on Wednesday and appointed Matthew Whitaker, who will serve as acting attorney general until the Senate confirms a permanent replacement.

Whitaker, who was once described as the West Wing's "eyes and ears" in the Justice Department, has previously made controversial remarks about both Mueller and the Russia investigation. He reportedly does not plan to recuse himself from overseeing the inquiry.

Read more: 'Seriously? This guy?': Matthew Whitaker's appointment as acting attorney general has FBI and DOJ officials in a 'daze'

"This was one of the biggest fears about Whitaker in law-enforcement circles," a former senior DOJ official who used to work with Mueller when Mueller was the FBI director told Business Insider. "His appointment not only endangers the status of the Russia investigation itself, but it could have an effect on the cooperation of those who have already pleaded guilty and others who Bob Mueller may be working to flip.

"It's hard to say whether that's what happened in Manafort's case without knowing more details," this person said. "But it's certainly a possibility. And that should set off some alarm bells."

Among other things, Whitaker wrote in a CNN op-ed article last year that Mueller had overstepped his mandate by digging into the Trump Organization's finances. He has said, without evidence, that there was "no collusion" between the Trump campaign and Russia. And audio recently resurfaced of Whitaker falsely accusing "the left" of sowing "this theory that essentially Russians interfered with the US election," a theory he claimed had been disproved and did not affect the election.

While he was Sessions' chief of staff, Whitaker met with Trump in the Oval Office more than a dozen times, The Washington Post reported, adding that whenever Trump complained about the Russia investigation, Whitaker "often smiled knowingly and nodded in assent."

NOW WATCH: Popular Videos from Insider Inc.