A Ukrainian maritime drone packed with explosives rammed into a Russian oil tanker early Saturday off the eastern coast of occupied Crimea, Russian officials and a Ukrainian official said, the second strike on a Russian ship at sea in two days.

That attack coincided with a new directive from Ukraine’s maritime authority, dated Friday, warning that six Russian Black Sea ports and the approaches to them would be considered "war risk” areas until further notice. The notice expanded on a less specific warning last month that any vessels sailing to ports in Russia or occupied Ukraine would be considered military targets.

Taken together, the tanker attack — which occurred in the Kerch Strait near a critical bridge connecting Russia and the Crimean Peninsula — and Kyiv’s new directive have ratcheted up the threat of expanded violence in the Black Sea. Tensions had been stoked by Russia’s sustained aerial assault on Ukraine’s ports since Moscow decided last month to withdraw from a U.N.-brokered deal allowing Ukrainian grain exports.