Attacks on merchant ships in the Black Sea have reached the waters of NATO members Bulgaria and Romania, raising fresh alarm about one of the world's important grain routes.

Around 3 a.m. local time on Tuesday, Oct. 6, sea and aerial drones struck two commercial vessels roughly 70 miles off the Bulgarian coastal town of Byala, Prime Minister Rumen Radev said. It was the first reported drone attack on shipping inside Bulgaria's exclusive economic zone, according to NBC News and The Kyiv Independent.

One ship sank, another burned

The Togo-flagged Alfa Watan suffered critical structural damage and sank at the scene, and there was no immediate word on its crew, Radev said. The second ship, the Palau-flagged Able, was carrying grain and caught fire. A Bulgarian ferry evacuated all 18 of its crew members, the BBC reported. Eleven were Turkish and seven were Indian, and two were badly injured and flown to a hospital in Varna.

Radev did not say who launched the drones and called the strike "a flagrant violation of maritime law." Ukrainian President Volodymyr Zelenskyy said Ukraine's military had no operations in the area and blamed Russia, saying the two navies had established that it was a combined Russian attack using sea and air drones. Moscow had not commented, according to the BBC.

A string of attacks

The Bulgarian incident came a day after a Turkish-owned cargo ship carrying corn from the Ukrainian port of Izmail toward Italy caught fire after a drone strike off Romania and later sank. Romanian authorities said two people died and 11 crew members were rescued, NBC News reported. Euronews reported that Azerbaijan's Foreign Ministry confirmed the ship's captain, an Azerbaijani national, was among the dead. Zelenskyy blamed that strike on Russian drones as well.

Also on Oct. 6, a Russian drone hit a Marshall Islands-flagged vessel carrying rapeseed oil, killing a steward and injuring seven others, according to the governor of Ukraine's Odesa region, as reported by The Kyiv Independent.

European Commission President Ursula von der Leyen called attacks on civilian shipping "unacceptable," and India, whose nationals were among the crews, condemned the strikes.

Turkey steps in

On Wednesday, Oct. 7, Turkish President Recep Tayyip Erdogan called Russian President Vladimir Putin and stressed the importance of safe navigation in the Black Sea, his office said. Erdogan also repeated that Turkey is ready to help arrange direct talks between Russia and Ukraine.

For shippers and grain buyers, the concern is that the danger zone is widening beyond waters near Ukraine and Russia into the western Black Sea, where vessels have generally sailed with less risk.