The vessel, which is Liberian-flagged, had not responded to repeated warnings when it entered an area Russia has described as dangerous for shipping. Although this is the first time this has happened, it could be a sign of rising tensions in the Baltic Sea.

The Green Admire left the port of Sillamäe in Estonia on Saturday evening and headed north towards the Gulf of Finland.

In the early hours of Sunday morning, the Russian authorities stopped the vessel and escorted it to an anchorage on the Russian-owned island Suursaar. The Russians warned the tanker several times that it was entering a dangerous area and ordered it to change course. Whether due to the language barrier or the fact that the tanker did not want to go further, the authorities' warnings were ignored.

"The Russians have declared certain sea zones dangerous for shipping. These areas are along their maritime border, in their territorial waters. In one place there is a section of the sea blocked off from where ships can sail into the port of St. Petersburg. This danger area has been created there, they have not enforced it in this way before, but if they want to enforce it in the future, they have every right to do so in their territorial waters," said Johan-Elias Seljamaa, deputy commander of the Estonian Navy.

However, the final decision on whether or not to pass through these dangers zones is made by the ship's captain. After the orders had been disobeyed, the Russian authorities stopped the vessel and anchored it in Suursaar. The island is used by Russia for military purposes, and is home to a large number of radar and other sensor networks.

"It has become such a militaristic, heavily fortified and well-equipped island. At the time when I went there by boat a few times, I visited Viiburi. I remember at night it was quite colorful. Because there were a lot of boats, a lot of spotlights and lots of questions and answers with morse code as well as other things. They were quite nervous there," said former Estonian Defense Forces (EDF) commander and marine scientist Tarmo Kõuts.

There is a safe navigation route in the Baltic Sea that does not follow any national borders and has been agreed upon between Russia, Estonia, and Finland.

However, since 2022, there has been an official recommendation in place advising ships to avoid maritime zones controlled by the Russian Federation. This advisory, agreed upon by Estonia, Finland, and Russia, was introduced amid rising geopolitical tensions and aims to minimize the risk of maritime incidents in contested or sensitive areas of the Baltic Sea.

While the use of sea routes in the Estonian zone is slightly more difficult, it is still perfectly feasible for large vessels.

"There are just more course changes. Vessels leaving the Estonian maritime area between the two shoals have done so in the past, and to my knowledge one vessel was even diverted through there today. There's nothing new in this respect, but given the size of the tankers, the limited depth, captains have found in the past that it is perhaps safer and better to use the other shipping route through the Russian maritime area," said Kristjan Truu, head of maritime services at the Transport Administration.

Although this is the first time Russia has made such a detention, similar situations occurring in the future cannot be ruled out as it aims to demonstrate its influence over the Baltic Sea.

"After all, the goods and the ship itself are all European. In other words, the owner of the ship and the goods are, of course, and given the fact that we are in a kind of trade war – this was done away with," Kõuts said.

When the tanker left Estonia it was carrying a cargo of shale oil and was destined for Rotterdam, the Netherlands, the Transport Administration said on Sunday afternoon. It is owned by the Greek company Aegean Shipping.

The Transport Administration monitors vessel traffic in Estonian waters daily. When leaving the port of Sillamäe, vessels transit the territorial sea of the Russian Federation, as this route is safer for large vessels than sailing through the shallows of the Estonian maritime area.

This safe trajectory for ships, which does not cross any national borders, has been agreed between Russia, Estonia and Finland.

In order to avoid similar situations occurring again in the future, the Estonian Transport Administration's vessel traffic management department, together with pilots, will direct ships to and from Sillamäe through Estonian waters.

Editor's note: This article was updated to report that Russian authorities had released the Green Admire to continue its journey to Rotterdam, and further clarification on the timeline of the advisory to avoid maritime zones controlled by the Russian Federation.

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