The vessel Sofia, sailing under a Russian flag, was likely responsible for a large oil slick discovered near Anapa at the end of last week, Igor Shkradyuk, a representative of the environmental project Prozrachnyi mir (Transparent World), told BBC Russia on April 17.

The spill of crude oil or heavy fuel oil was detected by the Sentinel-1 satellite and occurred on the evening of April 7. By April 14, the slick covered more than 200 square meters (2,153 square feet) and had begun to break apart. Part of the pollution was carried by wind toward the Utrish nature reserve.

Russia’s Marine Rescue Service deployed 3 vessels to clean up the spill, Shkradyuk said, adding that the number was insufficient to recover much of it.

Preliminary estimates suggest that between 300 and 350 tons of oil products may have entered the sea. About 28 tons of oil-containing mixture have been collected from the water’s surface, BBC Russia reported.

Krasnodar Krai’s operational headquarters had previously said that Ukrainian drone strikes on civilian vessels in the Black Sea and the Sea of Azov may have caused the spill. Waterfowl were killed or injured as a result.