WASHINGTON, Aug 15 (Reuters) - A former FBI agent admitted to working for Russian oligarch Oleg Deripaska while he was under U.S. sanctions and pleaded guilty to one count of conspiracy in a New York court on Tuesday, in exchange for prosecutors dropping other charges.

Prosecutors in January said Charles McGonigal, who led the FBI's counterintelligence division in New York before retiring in 2018, received concealed payments from Deripaska in exchange for investigating a rival businessman and unsuccessfully pushed in 2019 to have Deripaska's sanctions lifted.

At the time, McGonigal pleaded not guilty to four criminal counts including sanctions violations and money laundering.

McGonigal, 55, said in federal court in Manhattan on Tuesday that he worked for Deripaska between spring and autumn of 2021 to find negative information on Vladimir Potanin in an attempt to have the Deripaska rival put on the U.S. sanctions list.

The former FBI agent received $17,500 for that work, which was routed from Russia through accounts in Cyprus and New Jersey in an attempt to mask the source of the payments, he said.

McGonigal told the court he was "deeply remorseful" for his actions.

His attorney Seth DuCharme told reporters after the hearing that his client was treated fairly.

The conspiracy charge carries a maximum sentence of 5 years in prison. U.S. District Judge Jennifer Rearden is scheduled to sentence McGonigal on Dec. 14.

Matthew Olsen, who leads the U.S. Department of Justice's National Security Division, said in a statement that the case shows the DOJ's "resolve to pursue and dismantle the illegal networks that Russian oligarchs use to try to escape the reach of our sanctions and evade our laws."

U.S. prosecutors charged McGonigal as they ramped up efforts to enforce sanctions on Russian officials and police their suspected enablers following the Russian invasion of Ukraine in February 2022.

Deripaska, the founder of Russian aluminum company RusalRUAL.MM was among the two dozen Russian businessmen and government officials who Washington imposed sanctions on in 2018 in reaction to Russia's purported meddling in the 2016 U.S. election.

He and the Kremlin have denied any election interference.

Potanin, 62, is one of Russia's richest men and the president and chairman at Nornickel (GMKN.MM), the world's largest producer of palladium and refined nickel.

He was placed on the U.S. sanctions list in December 2022.