France said Thursday it had seized a superyacht owned by Russia's oil czar Igor Sechin, following through on threats to target sanctioned oligarchs close to Russian leader Vladimir Putin.

The estimated $120-million vessel "Amore Vero" was seized while undergoing maintenance work at a shipyard at La Ciotat on the French Riviera, a popular summer playground for the super-rich.

"French customs carried out the seizure of the yacht Amore Vero in La Ciotat as part of the implementation of sanctions by the European Union against Russia," a statement for the French finance ministry said.

The 85.6-metre (280 ft) vessel was owned by a company "in which Sechin is the main shareholder", the statement added.

Authorities moved after it "was preparing to cast off in a hurry, without the work on it having been finished," junior budget minister Olivier Dussopt said.

Sechin, seen as part of Putin's tightest inner circle and a former deputy prime minister, is chief executive of Russian oil giant Rosneft.

Another superyacht moored in Mediterranean port of Monaco, the "Quantum Blue", owned by Russian billionaire Sergei Galitsky, was also being checked by customs on Thursday, a source in the principality told AFP.

German authorities denied a report in Forbes magazine that they had seized the "Dilbar" yacht, belonging to billionaire businessman Alisher Usmanov, in the port of Hamburg.

Part of the challenge for European authorities is determining the ownership of vessels, which are usually registered in low-tax jurisdictions and sometimes owned by trusts or front companies.

Rory Jackson, business editor at the Superyacht Group, said that the boats being targeted were likely in Europe because they were undergoing maintenance.

"At this point in the year, it's the end of the Caribbean season and the Mediterranean season will start in May," he told AFP. "The yachts that are in Europe in this period, they're probably having refit work done."

He said there was evidence that Russian owners were keeping their vessels in the Caribbean or moving them to places out of reach of European or American authorities such as the Seychelles.

Authorities in the Maldives have told AFP that the "Clio" superyacht of billionaire Oleg Deripaska has anchored off its capital, along with the "Titan" vessel owned by steel magnate Alexander Abramov.

The megayacht Solaris owned by Roman Abramovich is currently shown as located in Barcelona in Spain, according to specialist ship tracking websites, and is thought to be undergoing maintenance.

Abramovich has not been sanctioned, but announced Wednesday that he was divesting from Chelsea football club in a possibly pre-emptive move.

The "Amore Vero" pictured in a shipyard in La Ciotat, near Marseille.
The "Amore Vero" pictured in a shipyard in La Ciotat, near Marseille. AFP / NICOLAS TUCAT

Putin himself has been repeatedly linked to the 80-metre boat "Graceful", which was moved from the German port of Hamburg to the Russian enclave of Kalingrad on the Baltic Sea in early February, ship-tracking website shows.

Around 500 Russian individuals have been targeted by sanctions from the European Union, with the the United States and Britain also announcing their own measures.

"Those who enable the invasion of Ukraine will pay a price for their action," EU foreign policy chief Josep Borrell said on Monday as he announced new sanctions after Russia's invasion of Ukraine last Thursday.

These included asset freezes for Sechin, pipeline boss Nikolay Tokarev, bankers Mikhail Fridman and Petr Aven and others.

Paris has said it is drawing up a list of assets in France owned by oligarchs including yachts and luxury cars.

"If I were an oligarch, in Russia or France, I'd be worried," French Foreign Minister Minister Jean-Yves Le Drian said on Monday.

US President Joe Biden has also tasked the Department of Justice with assembling a task force to "to find and seize their yachts, their luxury apartments their private jets."

The "Amore Vero" vessel seized by France on Thursday was made by Netherlands-based yacht builder Oceanco and includes a swimming pool that turns into a helipad.

Russians have long been major buyers of superyachts -- pleasure vessels classed as being more than 30 metres long.

The London-based Superyacht Group estimates they make up about 10 percent of the global market.

"It's not a majority, but its a significant portion," Jackson said.

The EU sanctions are expected to badly hit the businesses of European superyacht builders concentrated in the Netherlands, Germany, Spain and Britain, as well as people who serve as crew.

But campaigners have long argued that Putin depends on the loyalty of the tight group of businessman around him, and that targeting their wealth is a way of destabilising his regime.

"So far, the lists are quite short of which oligarchs are going to be sanctioned. They need to be much longer," campaigner Bill Browder told AFP this week.