ATHENS, Greece (AP) — A Greek-flagged oil tanker was damaged in a suspected drone attack in the Black Sea while approaching the Russian port of Novorossiysk, authorities said.

Greece’s shipping ministry said the attack occurred early Saturday, causing material damage, but the 24 crew members were unharmed and the vessel’s seaworthiness was not under threat.

The Maran Homer, chartered by U.S. oil giant Chevron, was traveling from Thessaloniki in northern Greece to the Black Sea port and was due to sail on to Istanbul. It was struck by a missile or drone before picking up Kazakh oil, according to Greek government officials and the vessel’s Greek operating company.

Greek Maritime Affairs Minister Vassilis Kikilias said Athens would lodge a “strong complaint” with the country deemed responsible, without giving further details. He later identified Ukraine as carrying out frequent attacks in the Black Sea.

“I consider the targeting of vessels flying the Greek flag — as well as those with Greek sailors and Greek shipping interests — to be unacceptable and extremely dangerous,” Kikilias told state-run ERT television.

He said he believed the attack could be related to a U.S. decision to temporarily ease some sanctions on Russian oil shipments because of volatility caused by the Iran war.

The war has also affected Greece’s massive shipping industry, with dozens of Greek-flagged or Greek-owned vessels currently stranded in the Persian Gulf.