The vessel, the Kiwala, had been sanctioned due to past activities by both the EU and at least three non-EU countries (Canada, Switzerland and the U.K.) and was en route to the Russian port of Ust-Luga when it was intercepted.

According to Estonian authorities, the vessel lacks a flag state, meaning it is not permitted to sail on the open seas.

"The purpose of the detention is to check the ship's documents and legal status. The detention and the investigations being carried out are in no way related to damage to critical infrastructure, and the inspection on board is being conducted by the appropriate authorities," Värk said.

Veiko Kommusaar, head of the Police and Border Guard Board's (PPA) border patrols, added: "It is no secret that over the past year, a number of vessels have appeared in the Gulf of Finland that sail without proper documentation. And it can be stated quite clearly that this is part of the shadow fleet."

Operation began at 3 a.m.

Estonian authorities began preparing for the operation on Thursday evening, and it got underway from 3 a.m. Friday, still before the Kiwala entered Estonia's exclusive economic zone.

Helicopter capabilities were used during the operation.

The vessel entered Estonian territorial waters at 4:19 a.m., sailing in a west to east direction, and had been en route from the port of Sikka, India, to Ust-Luga.

This likely means the tanker was empty at the time it was apprehended.

Kommusaar added: "The ship had no flag state. A stateless vessel. Ships like this are actually not allowed to operate. Estonia exercised its right to detain the vessel for inspection."

The Transport Administration (Transpordiamet) is inspecting the ship's technical condition, insurance, and the documents of its crew members, through the course of the inspection.

There are 24 people aboard, Cdre Värk said at a press conference Friday.

The ship's captain is reportedly a Chinese national and has been cooperative during the investigation so far. The remaining crew members of the ship are reportedly citizens of Myanmar.

The ship will be detained until the technical deficiencies are eliminated.

The Marine Traffic site earlie listed the tanker as east of the offshore Estonian island of Aegna, in Muuga Bay, and accompanied by two warships.

According to Ukraine's WarSanctions portal, the vessel is part of Russia's shadow fleet and is involved in exporting Russian crude oil and oil products to third countries. The portal reports that the ship's owners and operators have changed frequently, to avoid detection of direct ties to Russia and to conceal the true beneficiaries.

WarSanctions states that the Kiwala sails under the flag of Djibouti.

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