Romanian authorities launched a search and rescue operation off the country's Black Sea coast on Thursday after a ship caught fire, leaving eight people missing and four others rescued.

The cause of the fire aboard the San Marino-flagged chemical tanker Wine Trader, which was travelling from Tunisia to Romania, was not immediately known. However, a series of recent drone attacks on vessels in the Black Sea has raised concerns that Russia's war in Ukraine could spread further across the region.

Romania's Ministry of National Defense said a group of aerial targets was detected overnight near Ukraine's Snake Island. A drone briefly entered Romanian airspace at 1:34am southeast of Sulina before disappearing from radar.

The Romanian Naval Authority said 12 people were aboard the tanker when the fire broke out about 13 nautical miles (15 miles) southeast of the port town of Sulina.

Similar drone incursions have occurred this month in the exclusive economic zones of Romania and Bulgaria, both members of NATO and the European Union.

On Wednesday evening, a fire broke out aboard the Comoros-flagged chemical and oil tanker Yara J, which was travelling from Turkey to Sulina, in Romanian territorial waters. All crew members were safe.

Romania's acting Defence Minister Radu Miruta said an investigation was underway, adding that military radars had not detected any drone approaching the tanker.

On Tuesday, drone attacks on two vessels off Bulgaria triggered emergency rescue operations. Eleven crew members of the Guinea-Bissau-flagged cargo ship HS Dignity remain missing.

The other vessel, Chelsea 3, a Ukrainian-operated coastal freighter sailing under the Maltese flag, was struck by three aerial drones, according to its crew. Its deck and hull were damaged.

Rescuers evacuated all 15 crew members from the vessel, five of whom received treatment for minor injuries.

Bulgarian Prime Minister Rumen Radev said Russia was likely responsible for Tuesday's attacks.

The incidents 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 rescued the 18-member crew of a damaged Palau-flagged vessel.

On October 5, a Turkish-owned cargo ship carrying corn caught fire following a drone attack and later sank in the Black Sea off Romania's coast. Two people were killed, while 11 crew members were rescued.