Celebrity defense attorney Alan Dershowitz and former FBI Director Louis Freeh have registered to lobby the U.S. government on behalf of an Israeli businessman who has been accused of getting rich from corrupt mining deals in Africa, according to a registration form reviewed by CNBC.

The businessman, Dan Gertler, who has a net worth of just over $1 billion, was sanctioned in 2017 and 2018 by the Trump administration for the alleged abuses in affiliation with his business dealings in the Democratic Republic of the Congo.

Lobbying is an unusual role for Dershowitz, who is one of President Donald Trump's most vocal advocates in the media and has famously defended high-profile clients such as O.J. Simpson. There were no other publicly available records that list Dershowitz as a registered lobbyist, according to a CNBC review. Dershowitz, however, insists he's acting as an attorney for Gertler.

"I am acting as a defense lawyer, not a lobbyist. But the rules require registration if you present a defense to a non-judicial agency," he told CNBC. Dershowitz and Freeh are lobbying the Treasury Department, which enacted the sanctions, on Gertler's behalf.

The filing shows that at least part of their lobbying efforts for Gertler started in October 2018. They appear to have officially registered to act on his behalf on Monday, the date signed on the document by Freeh's partner, Gregory Paw.

Gertler, according to the Treasury Department, "has amassed his fortune through hundreds of millions of dollars' worth of opaque and corrupt mining and oil deals" in the DRC. The Treasury put a freeze on his U.S. assets two years ago. In 2017, Gertler sold his stake in two Congo mines to Glencore, a commodities trading company based in Switzerland, for more than $960 million.

The lobbying disclosure form shows that Dershowitz, Freeh and their associate, Gary Apfel of law firm Pepper Hamilton, are targeting the Treasury and, specifically, the Office of Foreign Assets Control. The document describes their lobbying push as a focus on ​"extractive industry & charitable activity."

Freeh and Dershowitz have known each other for years. After Dershowitz was accused of sexual misconduct by a woman who has claimed she was abused by late financier Jeffrey Epstein, he hired Freeh's security firm to conduct an independent investigation. Freeh, in 2016, announced that his team "found no evidence to support the accusations of sexual misconduct" against Dershowitz.

"In my opinion, the totality of the evidence found during the investigation refutes the allegations made against Professor Dershowitz," Freeh said in a statement at the time.

Dershowitz was a staunch defender of Trump during special counsel Robert Mueller's investigation and has become an advocate for the president during the impeachment inquiry into the July 25 phone call Trump had with Ukrainian President Volodymyr Zelensky.

The New York Times reported in December that Dershowitz was acting as an advisor for Gertler as he fought off the sanctions. Dershowitz told the Times that Gertler is a "very good person" and is "being targeted primarily because of the actions of other people." He said at the time he was not using his access to Trump to lobby to the administration on Gertler's behalf.

"I would never raise an issue like this," he told the Times.

Dershowitz once defended Epstein, who was accused of child sex trafficking before he was found dead in his jail cell from an apparent suicide in August. The two were also friends.

Freeh started using his law firm, Freeh Sporkin & Sullivan, to lobby for clients earlier this year. As Politico reported at the time, one of Freeh's first clients was Kuwait-based KGL Investment Company. His team was reportedly pushing for the release of Marsha Lazareva, a KGL executive who had been detained in that country since 2017.

Apfel and a representative for Freeh did not return a request for comment. Calls to Gertler's office in Israel were not returned.