NEW YORK (NYTIMES) - As his steel mill in Mariupol, Ukraine, was being destroyed by Russian bombs, becoming the site of a last stand by the city's defenders, billionaire Rinat Akhmetov, Ukraine's richest man, emerged as a staunch supporter of the country's war effort.

But even as Akhmetov pledged to help rebuild Ukraine, a large portion of his fortune was committed to a secret vanity project that speaks to his past as a pro-Russian oligarch: A German shipyard is building a superyacht for his family's use.

The yacht, now known only by the code name Luminance, may be among the 10 largest superyachts in the world when finished. Nearly half of those are owned by Russian business magnates. In tonnage, the 144m luxury vessel could approach the size of the Moskva, the Russian navy cruiser that the Ukrainian military said it sank last month in the Black Sea.

Now, Akhmetov is considering selling the yacht, even before it is completed. "Given the current circumstances, we are considering its possible sale," Jock Mendoza-Wilson, director of international relations for Akhmetov's company, System Capital Management, said in a statement. The New York Times is the first to reveal Akhmetov's ownership of the vessel.

Owning a luxury yacht has become a status symbol for oligarchs from Russia and other former republics of the Soviet Union, who in recent years have eclipsed Gulf state kings, princes, sultans and sheikhs as the top buyers of the very biggest vessels.

But since Russia's 2014 annexation of Crimea and its support of a separatist movement in eastern Ukraine, Akhmetov has distanced himself from his pro-Russian past, which included financial backing for the Kremlin-supported and former Ukrainian President Viktor Yanukovych. Akhmetov's purchase of the Luminance is a throwback to that world.

Luminance is being built by Lürssen, a German company that also built the Scheherazade, a superyacht that United States officials say is linked to Russian President Vladimir Putin. Italian authorities announced this month that they had impounded that yacht. The two vessels also share the same construction manager, Imperial Yachts, a Monaco-based company that counts many Russian oligarchs as customers.

But Luminance is about 4m longer than the US$700 million (S$966 million) Scheherazade. That kind of money is almost enough for Ukraine to buy two new German-built attack submarines and is about the cost of a top-of-the-line Baden-Württemberg-class naval frigate, a warship armed with guns and missiles that Lürssen also helps to build. A spokesperson for Lürssen did not comment.

System Capital Management controls companies in a wide range of industries, including television stations, wind farms, electrical grids, chemical plants and the now-famous Azovstal steel plant in Mariupol.

Mendoza-Wilson said Akhmetov had signed the contract to build the yacht in 2018. But the work continued well after the Russian invasion began in late February, according to a person who has worked at one of the many companies with contracts to help build the vessel. The employee, who asked for anonymity because he did not want to subject his colleagues to reprisals, said some of Akhmetov's relatives had been making trips in Western Europe to go over construction details. Akhmetov has been in Ukraine during the war.

It is unclear how Akhmetov could sell a vessel that is still under construction or back out of the contract that he signed. But a person who is working on the Luminance project, speaking anonymously because he signed a nondisclosure agreement, said the best time for him to sell might be when the ship is finished and ready to be delivered, because a potential buyer could take possession of a new yacht without the hassles involved in negotiating contracts and years of waiting while the vessel is being built. One industry estimate said the ship would be done next year.

"The best time to sell a boat is the day before it launches," the person said.