For the first time, the French Navy has deployed the 101-meter-long intelligence ship Dupuy de Lôme to the Baltic Sea to monitor Russia’s shadow fleet of oil tankers.

The mission’s goal is to intercept radio signals related to Russian activities — including monitoring “shadow” tankers that are circumventing the EU’s oil price cap sanctions.

Tensions have risen dramatically in the last week after the Estonian navy attempted to detain another shadow fleet tanker attempting to reach the Russian port of Primorsk. Russia sent an SU-35 fighter jet to escort the ship into port in the first time Russia’s military has faced off directly with the naval forces of a Nato member.

Also on the same day a sanctioned shadow fleet tanker was carrying out “suspicious manoeuvres” that might threaten an undersea power cable linking Poland with Sweden, Polish Prime Minister Donald Tusk said in a post on X on May 21.

"After the effective intervention of our military, the ship sailed to one of the Russian ports," he added. He said the Polish Navy's ORP Heweliusz was sailing to the scene.

The 600-megawatt undersea cable links the Swedish coast near Karlshamn with Ustka in northern Poland and allows for cross-border supplies of power when electricity is cheaper in the other system, Reuters reports.

The EU introduced a seventeenth sanctions package on May 20 that widens the number of shadow fleet tankers singled out for restrictions to a total of 350. However, as under international maritime law ships enjoy the “innocent right of passage” through another country’s waters without having to seek permission. If an EU country uses its navy to stop shadow fleet tankers for inspection, or any other reason, then that is deemed a “naval blockade” under international law, which is an act of war. US president John F Kennedy faced a similar dilemma when he was trying to prevent Soviet ships from delivering missiles to Cuba in 1962, hence setting up a naval “quarantine” around Cuba instead.

Russia has vowed to defend its shipping interests in the Baltic Sea using “all means” at its disposal after the Estonian attempt to stop a Russian-bound oil tanker prompted a brief one-minute Nato airspace violation by the Russian fighter jet and the subsequent detention of a Greek-owned vessel.

Kremlin spokesman Dmitry Peskov said the same day that Moscow was prepared to act “decisively” following what it called a “pirate attack” by Estonia.

“As the latest events related to the attempted pirate attack on one of the tankers showed, Russia demonstrated it is capable of responding quite harshly,” Peskov told reporters. He added that Russia was ready to use “all means” within international law and had a broad range of options available to protect its vessels.

Following the Estonian navy incident, in apparent retaliation, a few days later Russian authorities detained a Greek-owned oil tanker on May 18 that had departed from an Estonian port. The vessel has since been released, but the arrest of the ship is another click in the ratcheting tensions in the Baltic Sea.

The deployment of the French intelligence ship, which is part of the French navy, but pointedly not a battleship, will only increase tensions further.

French defence officials confirmed the deployment of the spy ship, which is equipped with radar and electronic tracking systems, is to better follow the tankers that are listed to obscure ownership, sail under flags of convenience, and often disable their transponders to evade detection.

France’s involvement marks a rare public move by a major Western military power in the enforcement of maritime sanctions in the Baltic. Paris has not disclosed the duration or scope of the surveillance mission, but officials said it is aimed at increasing intelligence-gathering capabilities and supporting EU monitoring efforts.

Western officials estimate that Russia is using over 600 tankers globally to bypass sanctions, many of which operate without proper safety protocols or insurance coverage.