A judge in Fiji is due to rule Tuesday on whether United States authorities can seize the luxurious superyacht Amadea — worth some US$325 million ($500m) — which has been stopped from leaving the South Pacific nation because of its links to Russia.

But a vital question remains over which oligarch really owns the Amadea. Only one of the two possible candidates faces sanctions.

Is the real owner Suleiman Kerimov? That's what the United States claims.

Kerimov, an economist and former Russian politician, was sanctioned by the United States in 2018 for alleged money laundering and has faced further sanctions from Canada, Europe and Britain after Russia invaded Ukraine.

Kerimov made a fortune investing in Russian gold producer Polyus, with Forbes magazine putting his net worth at US$14.5 billion ($22.2b).

Or is the real owner Eduard Khudainatov? That's what defence lawyers claim.

Khudainatov is the former chairman and chief executive of Rosneft, the state-controlled Russian oil and gas company.

Crucially, Khudainatov currently does not appear to face any sanctions, unlike many oligarchs and people with close ties to Russian President Vladimir Putin who have been sanctioned since the war began.

As with many superyachts, determining the real ownership of the Amadea is difficult due to the shadowy trail of trusts and shell companies.

On paper, the superyacht is registered in the Cayman Islands and owned by Millemarin Investments Ltd, also based in the Cayman Islands.

Defence lawyers have claimed in court that Millemarin Investments Ltd (sometimes spelled Millemarine) is the legal owner of the vessel and that the company is linked to the real, or beneficial, owner, Khudainatov.

Russian billionaire Suleyman Kerimov. Photo / Getty

But US authorities have claimed that behind all the various fronts, the real owner is Kerimov.

On April 19, after the yacht had sailed into Fiji from Mexico, the High Court in Suva ordered that the Amadea not leave Fiji until the merits of the American warrant to seize the vessel were determined.

Perhaps reflecting the question over ownership, the court later ordered Fijian prosecutors to amend an original summons which named just Kerimov to also include Millemarin Investments Ltd as a second respondent to the case.

For now, the yacht continues to sit in a Fijian harbor with its crew of about 25 rotating on and off the vessel, while a police officer remains on board to ensure it stays put.

According to Boat International, the Amadea is 106m long and was built in 2017.

It features a stainless steel albatross that extends off the bow and weighs more than 5 tons, a live lobster tank in the galley, a 10m pool, a hand-painted Pleyel piano and a large helipad.

The US Embassy in Suva said in a statement that the United States was acting with allies and partners around the world to impose costs on Russia because of its "war of choice."

"We continue to ratchet up the pressure on Putin's oligarchs and we are working with allies and partners to go after corrupt gains from some of the individuals closest to Putin, no matter where they are held around the world," the embassy said.

-AP