Russia detained a Greek-owned, Liberia-flagged oil tanker, the Green Admire, on Sunday after it left Estonia’s Sillamae port and crossed a navigation channel through Russian waters.

Estonia said the detention may be retaliation for its actions against Russia’s shadow oil fleet and has alerted NATO.

The tanker, carrying Estonian shale oil to Rotterdam, was reportedly moved by Russia to impose a fine.

The incident comes amid heightened tensions in the Baltic Sea, where infrastructure sabotage fears are growing, Reuters has reported.

Greece is monitoring the situation but has not been asked to intervene.

Written by B.C. Begley