The fallout from a week of drone attacks on ships in the Black Sea is spreading. An oil slick covering about 25 square kilometers, or 10 square miles, formed after sea drones attacked an oil tanker off the Russian resort city of Sochi, Greenpeace said Thursday after studying satellite images, Meduza reported.

The Liberian-flagged tanker caught fire on October 6 while carrying crude oil from the Russian port of Novorossiysk to India, according to early reports. All 23 crew members were evacuated. Russia's Transport Ministry blamed uncrewed boats for the attack.

Russian officials have played down the damage. The country's consumer safety watchdog said on October 7 that there was no pollution in the sea, and Sochi's mayor said no oil had been detected, though swimming limits were put in place on the city's beaches. Kremlin spokesman Dmitry Peskov said cleanup work was "going well."

Farther west, the search for missing sailors has ended. A drone struck the Togo-flagged cargo ship Alfa Watan about 70 nautical miles off Bulgaria's coast early on October 6, and the ship sank in about three minutes, Bulgarian officials said. Rescuers found an overturned boat and three life rafts but no trace of the crew, and Bulgaria called off the search on Wednesday, Novinite reported.

Nine Syrians and one Egyptian were aboard, according to The National. Syria on Thursday called for an investigation by Bulgaria and the International Maritime Organization into why the search was stopped before the crew's fate was known. A second ship hit the same night, the Able, was abandoned on fire, but its crew was rescued.

No one has claimed the attack off Bulgaria. Prime Minister Rumen Radev said the cargo and the ships' routes point toward Russia, but that Bulgaria cannot blame Russia without proof. Russia's embassy in Sofia denied any role, and Ukraine's president blamed Russia, The National reported.

Bulgaria plans talks with Turkey and Romania on protecting shipping and may ask NATO allies for more surveillance and counter-drone systems.

What we know: An oil slick has formed off Russia's coast, and 10 sailors from the Alfa Watan are still missing.
What we don't: Who attacked the ships off Bulgaria.