Greece has issued a warning to its shipping fleet to reassess security measures when sailing to Russian Black Sea ports following drone attacks on two Greek-operated oil tankers earlier this week, according to shipping ministry advisories.

On Tuesday, drones targeted two tankers en route to a Black Sea terminal on Russia’s coast, including one chartered by U.S. oil major Chevron. Greek shipping companies operate some of the world’s largest fleets of tankers, which play a crucial role in trade across the Black Sea, a region shared by Bulgaria, Georgia, Romania, Turkey, and the conflict zones of Russia and Ukraine.

“It is recommended that the security managers of shipping companies, the masters and the security officers of Greek ships proceed with an updated threat assessment for commercial ships located in the Black Sea and the maritime areas close to it,” the ministry said in one advisory seen by Reuters.

War insurance costs for vessels in the Black Sea have risen sharply this week, reflecting the heightened risk to commercial shipping. One of the tankers struck on Tuesday, the Matilda, managed by Greece’s Thenamaris, was hit by two drones. While no injuries or serious damage were reported, Thenamaris said it has implemented additional precautionary measures and instructed crew to remain vigilant and avoid unnecessary deck movement.

The ministry also referred shipping companies to a previous April 2022 document recommending enhanced protective measures for Russian Black Sea ports, including Novorossiysk, Taman, Tuapse, and Kavkaz, due to persistent security risks.