An American lobbyist who worked with Russian and Ukrainian oligarchs pleaded guilty Friday to failing to register to work as an agent of a foreign power and agreed to cooperate with federal prosecutors, widening the already broad reach of the special counsel.

The lobbyist, Sam Patten, has worked in a similar world as President Donald Trump’s ex-campaign chairman, Paul Manafort, who was recently convicted of financial fraud. Mr. Patten had acted this year as a foreign agent for the Opposition Bloc, a pro-Russia Ukrainian political party, according to a court document filed Friday. Mr. Manafort worked for the Opposition Bloc in 2014.

The charge against Mr. Patten stems from the special-counsel investigation of Russia’s interference in the 2016 U.S. presidential election, which includes a case against Mr. Manafort. But Mr. Patten’s case was handled by prosecutors outside the special counsel’s team in federal court in Washington. Part of his guilty plea included an agreement that he would cooperate with federal prosecutors.

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Mr. Manafort has also been charged with failing to register as a foreign agent for his work on behalf of pro-Russian Ukrainian interests, including the Opposition Bloc, and that trial is scheduled for September in Washington.

Mr. Patten’s lawyer declined to comment after Friday’s hearing. Mr. Patten faces up to five years in prison and a fine of up to US$250,000. He was released after the hearing and awaits sentencing.

For decades, prosecutors have rarely brought charges for violating the Foreign Agents Registration Act. But that changed, even before the appointment of the special counsel, as the Justice Department worked to meet the counterintelligence threats from foreign adversaries.

The special counsel, Robert Mueller, has pursued other investigations into lawyers and lobbyists who did work in Ukraine. Mr. Mueller has handed off some of these cases to federal prosecutors in other jurisdictions. A lawyer working for the special counsel was present in the courtroom Friday when Mr. Patten entered his guilty plea.

Mr. Patten’s work for the Opposition Bloc was done in conjunction with Konstantin V. Kilimnik, a Russian national who served as Mr. Manafort’s longtime deputy on the ground in Kiev. Mr. Mueller and his team have accused Mr. Kilimnik of having ties to Russian intelligence. Mr. Kilimnik was indicted in June on charges of obstruction of justice related to witness tampering in the foreign-lobbying case against Mr. Manafort.

Mr. Kilimnik is not named in Friday’s charging papers against Mr. Patten, but is identified as “Foreigner A,” a Russian national who formed a company in the United States with Mr. Patten. That company is identified as “Company A” in the court document. Mr. Patten and Mr. Kilimnik created a company called Begemot Ventures International in February, 2015, according to corporate records filed in Washington.

According to the charging document, “Company A” was paid more than US$1-million to assist the Opposition Bloc and its members, including a “prominent Ukraine oligarch,” who is not named.