The cause of the fire on Thursday aboard the San Marino-flagged Wine Trader, a chemical tanker travelling from Tunisia to Romania, was not immediately clear, but a recent spate of drone strikes on ships in the Black Sea has raised fears of Russia's war in Ukraine spreading.
A "group of air targets" was monitored overnight near Ukraine's Snake Island, and a drone briefly entered Romania's airspace at 1.34am southeast of Sulina, before disappearing from radar, according to Romania's defence ministry.
There were 12 people aboard the ship when the fire broke out 24km southeast of the port town of Sulina, according to the Romanian Naval Authority, with four of them rescued.
Similar drone incursions have occurred in October in the exclusive economic zones of Romania and Bulgaria, both NATO and European Union members.
A fire was reported on the Comoros-flagged Yara J, a chemical and oil tanker travelling from Turkey to Sulina, Romania, on Wednesday evening in Romanian territorial waters. The crew was safe.
Romanian Acting Defence Minister Radu Miruta said an investigation was under way but that no drone was seen on military radars heading towards that tanker.
On Tuesday, drone strikes on two ships off Bulgaria prompted emergency rescue operations for 11 crew members from the HS Dignity, a Guinea-Bissau-flagged cargo ship, who remain missing.
The other ship, a Ukrainian-operated coastal freighter named Chelsea 3, was sailing under a Maltese flag.
Rescuers evacuated all 15 crew members, with five receiving treatment for minor injuries.
The crew reported the ship's deck and hull were hit by three aerial drones.
The Tuesday incidents, for which Bulgarian Prime Minister Rumen Radev said Russia was likely responsible for, came a week after sea and aerial drones struck two other commercial vessels in Bulgaria's exclusive economic zone in the Black Sea.
A Togo-flagged ship sank, while a Bulgarian ferry evacuated the 18-member crew of a damaged Palau-flagged vessel.
A day earlier, on October 5, a Turkish-owned cargo ship carrying corn caught fire following a drone attack and later sank in the Black Sea off the coast of Romania.
Two people died and 11 crew members were rescued.
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