Ukrainian drones struck Russia’s Black Sea port of Tuapse on Friday for the fourth time in 16 days as authorities struggled to cope with a mounting environmental disaster from toxic black smoke clouds and oil leaking into the sea.

Ukraine’s SBU security service said drones had again struck the sea port and refinerythat make Tuapse an important hub for Russian oil exports.

Reuters could not confirm the latest strike on the refinery, which has been hit and set ablaze at least twice since April 16 in previous attacks that have halted production.

Local Russian officials said a major operation was under way to put out a fire at the sea port.

The attacks have thrown up dense black clouds over the town and caused oil slicks along the coastline, ruining the beaches of the popular resort.

They are part of what President Volodymyr Zelenskiy said was a Ukrainian strategy to disrupt Russia’s huge energy industry and knock key sites out of operation for as long as possible.