Russia's Federal Security Service (FSB) said on Monday that it had found traces of explosives on a ship travelling from Turkey to the port of Rostov-on-Don in Russia to pick up grain.
The FSB said the ship had been docked in the Ukrainian port of Kiliia in May, and that it may have been used to deliver explosives to Ukraine.
It said the ship had changed its name while in the Turkish port of Tuzla earlier this month and replaced its crew, which had consisted of 12 Ukrainian nationals.
"These circumstances may indicate the possibility of using the foreign civilian ship to deliver explosives to the territory of Ukraine," it said.
The vessel, which it did not name, had been inspected in the Kerch Strait and was banned from continuing its journey, after which it left Russian waters, the FSB said.
Reuters could not immediately confirm the details of the FSB statement. It came one week after Russia pulled out of an agreement that had enabled Ukraine to export grain from its Black Sea ports, with ships undergoing security inspections.