(Bloomberg) -- Greece’s Navy extended a series of exercises aimed at preventing tankers from transferring Russian oil just miles off its coastline.

The navy said in a notice that it will be conducting exercises until May 19 in the Laconian Gulf, an expanse of water that’s become well-known in the oil market as a location for Russian oil switching. It had been due to finish on May 9.

A person familiar with the matter said that the exercises are aimed at deterring the ship-to-ship transfers. When Greece first announced the drills last week, tankers quickly evacuated the area, gathering to the south instead. 

Read more: Greek Naval Drills See Oil Tankers Depart Key Transfer Area

The gulf, surrounded by hills that protect ships from the wind, has become a pivotal location for the shadowy transfer of Russian cargoes for onward shipment to buyers in Asia. 

However, because the transfers are happening outside of Greece’s direct territorial waters, Athens hadn’t been able to prevent the switching. It’s not clear what has changed for Greece to take a more-assertive approach now.

The transfers have sparked concerns of a major environmental spill in a picturesque area of European coastline. 

Denmark, which is next to a trade route for Russian oil from the Baltic Sea, also carried out similar, albeit brief, naval activity about a month ago.

When Bloomberg News visited the Laconian Gulf in September last year, about a dozen tankers were involved in cargo switching. 

Some of them were part of a large so-called shadow-fleet of aging tankers whose ownership and insurance status was unclear, raising a question about who would be liable were something to go wrong during the transfer.

--With assistance from Paul Tugwell.

(Updates with context about the Laconian Gulf)

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