A European naval mission boarded a Russia-linked oil tanker suspected of sailing under a false flag in the Mediterranean over the weekend, the EU’s top diplomat Kaja Kallas said Monday.

Personnel operating under the Mediterranean naval initiative Operation EUNAVFOR MED IRINI boarded the MV SUN on Sunday for “flag verification,” Kallas wrote in a post on X.

“This is the sixth suspected shadow fleet vessel boarded by our EU operations in recent months,” she said, referring to vessels with opaque ownership used to bypass Western sanctions against Russia.

She said EU defense ministers plan to discuss “actions against the shadow fleet” at their meeting in Ireland on Tuesday.

In July, the EU adopted a mechanism allowing member states to confiscate and sell Russian oil carried aboard shadow fleet vessels attempting to evade the bloc’s sanctions.

President Vladimir Putin this month vowed to retaliate in kind if European countries carried out plans to seize Russian commercial vessels.

Putin, who previously said the Russian Defense Ministry had received instructions to prepare a response, has slammed previous interceptions of Russia-linked vessels as “piracy.”

EUNAVFOR MED Irini is an EU naval mission first ​launched in 2020 to enforce a UN arms embargo on Libya. EU governments have since broadened its scope, authorizing ​it to conduct verification boardings.

The EU has sanctioned around 600 ships suspected of being part of Russia’s “shadow fleet” network.

Reuters contributed reporting.