The United States is seeking the forfeiture of a $US300 million ($A475 million) superyacht it says is controlled by billionaire Russian oligarch Suleiman Kerimov, who is under US sanctions. Authorities in Fiji seized the 106-metre Amadea yacht pursuant to a US warrant in May 2022 as Washington ramped up sanctions enforcement against people close to Russian President Vladimir Putin, to pressure Moscow to halt its war against Ukraine. Monday's complaint, filed in federal court in Manhattan, kicks off a potentially long judicial process in which the United States would seek ownership of the yacht, which is docked in San Diego, and then likely auction it and transfer proceeds to Ukraine. Kerimov and his family are worth $US10.7 billion, according to Forbes magazine. He amassed much of his wealth through a stake in Russian gold producer Polyus. Kerimov was sanctioned by the US Treasury Department in 2014 and 2018 in response to Russia's activities in Syria and Ukraine. Those sanctions barred Kerimov from accessing the US financial system. Polyus was sanctioned in May 2023. The company said the sanctions were unfounded. In Monday's complaint, the US Department of Justice said Kerimov bought the Amadea in 2021, and then violated US sanctions by making more than $US1 million in maintenance payments through US financial institutions. The yacht's owners will have the chance to contest that claim in court. Kerimov could not immediately be reached for comment. Lawyers for Millemarin Investments, a company that says it owns the Amadea, told a Fiji court last year that the Amadea was owned not by Kerimov but by former Rosneft chief Eduard Khudainatov, a Russian oligarch who has not been sanctioned. Khudainatov is not named in Monday's complaint. US prosecutors said a September 14, 2021, transaction transferring ownership of the Amadea from Millemarin to a newly incorporated company, Errigan Marine, was designed to make it appear that Evgeny Kochman, the president of sanctioned yacht broker Imperial Yachts, owned the yacht. Prosecutors said Kochman was, in fact, only a "straw owner." In a statement, Imperial Yachts said it disputed the DOJ's allegations and that neither the company nor Kochman "engaged in any illegal acts." Khudainatov sued the US on Monday in federal court in San Diego to release the yacht, according to a copy of the complaint provided by his lawyer, Adam Ford. Reuters could not immediately locate the legal papers online. "The Amadea was seized upon false premises driven by political motivation," Ford said in a statement. Australian Associated Press