Fiji police are investigating the arrival of a luxury vessel owned by a Russian oligarch, questioning its captain about how he brought the boat to the Pacific island nation without customs clearance.

Fiji newspapers reported that police had seized the superyacht Amadea, owned by a Russian billionaire, Suleiman Kerimov, who has been sanctioned by the United States, Britain and the European Union over Russia's invasion of Ukraine, and detained its crew.

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, lawmakers and businessmen have faced wide-reaching sanctions in the wake of 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 the Amadea had docked in Fiji's Lautoka port and the embassy was cooperating 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 in a statement.

"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, Brigadier General Sitivini 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 European Union delegations in Fiji had also requested cooperation, the Fiji Times reported.

Fiji had been alerted to the approach of the 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.

The Marine Traffic website showed Amadea left Mexico 18 days ago.

- Reuters