Ukraine accused Russia on Thursday of using strategic bombers to strike a civilian grain vessel in a missile attack in Black Sea waters near NATO member Romania, escalating tensions between Moscow and the military alliance.

President Volodymyr Zelenskiy said the vessel carrying Ukrainian grain to Egypt had been hit overnight by a Russian missile just after it left Ukrainian territorial waters. There were no casualties, he said.

The U.S. ambassador to Ukraine “strongly condemned” the attack and said Russia was responsible. A United Nations spokesperson said the incident was a “stark reminder” of the threats still faced in the Black Sea by civilian vessels.