A former Obama White House counsel is facing increasing pressure from federal prosecutors over his former law firm’s ties to Paul Manafort’s lobbying work for the Ukrainian government, The Washington Post reported.

Federal prosecutors reportedly are examining whether Washington attorney Gregory Craig, 73, failed to register as a foreign agent. According to court filings, Manafort helped pay Craig's former law firm -- Skadden, Arps, Slate, Meager & Flom LLP -- $4 million to review the prosecution of former Ukrainian president Viktor Yanukovych’s rival Yulia Tymoshenko in 2012.

Craig’s attorneys have downplayed the extent of his involvement in Manafort’s lobbying campaign, arguing that the activity did not warrant registering as a foreign lobbyist.

Craig, who left Skadden in April, was among two other men – Washington lobbyists Vin Weber and Tony Podesta – who were referred to the Southern District of New York that month, The Post reported.

Court filings allege that Weber, a former Republican congressman, and Podesta, a Democratic lobbyist, were also involved in Manafort’s lobbying efforts for Ukraine. The filings named two companies that matched descriptions of Weber’s Mercury Public Affairs and Podesta’s lobbying firm Podesta Group, respectively, which received $1 million to lobby on behalf of the Ukrainian government in 2012, the report said.

According to the report, both firms failed to register as a foreign agent. Representatives for Weber and Podesta have argued their clients did not need to.

The three men’s cases were referred to New York prosecutors in April amid special counsel Robert Mueller’s investigation into Russian interference in the 2016 presidential election. No charges have been filed yet against either of the three men as prosecutors review their cases.

Manafort, who was briefly President Donald Trump’s campaign chairman during the 2016 election, pleaded guilty earlier this month to charges of conspiracy and obstruction in exchange for cooperation with Mueller’s team.