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Fiji is investigating the arrival of the luxury superyacht Amadea, which is thought to be owned by Russian billionaire Suleiman Kerimov.
Police are questioning its captain on how he brought the boat to the Pacific island nation without Customs clearance.
Fiji newspapers reported that police had seized the 106-metre vessel Amadea, which is thought to be worth $325 million, and detained its crew.
A superyacht agent in Fiji, who said he represented Amadea, told Reuters the reports were “not fully factual including ownership of the vessel “.
“I do not know who the owner is. There is evidence being released to the police in Fiji,” the agent, Chase Smith of Baobab Marine, told Reuters.
He said two sets of lawyers were handling the situation and he continued to act for Amadea and its crew.
Mr Kerimov has been sanctioned by the US, UK and EU over Russia's invasion of Ukraine.
An official at the National Police Command and Control Centre confirmed to Reuters the captain of the vessel, which arrived on Tuesday, was being questioned about how it came to Fiji without Customs clearance.
Russian President Vladimir Putin, country's lawmakers and businessmen have faced wide-reaching sanctions after the invasion, which Moscow calls a special military operation, while European countries have seized property including villas and boats.
The US Embassy said it was aware Amadea had docked in Fiji's Lautoka port and the Embassy was co-operating with Fijian authorities on the matter.
“The United States is committed to finding and seizing the assets of the oligarchs who have supported the Russian Federation’s brutal, unprovoked war of choice against Ukraine,” the Embassy said.
“We and several of our EU partners have already frozen or seized many assets of these oligarchs. We are working closely with governments and private sector partners in Europe, and the entire world, including Fiji, on this issue.”
Commissioner of Police, Brig Gen Sitiveni Qiliho, told the Fiji Sun newspaper the Attorney General's office had been contacted by a foreign government requesting assistance in a criminal matter, and Fiji had agreements with other countries to enforce sanctions on Russian oligarchs.
The EU delegations in Fiji had also requested cooperation, the Fiji Times reported.
Fiji had been alerted to the approach of Amadea before it moored at Lautoka Wharf, the media reported.
An official in Attorney General Aiyaz Sayed-Khaiyum's office declined to comment saying it was an internal matter for the Fiji government.
Superyachts entering Fiji are required to apply for health clearances before departing the previous port and submit other Customs documents three days before arriving in Fijian waters.
