A Russian warship fired warning shots with automatic weapons on Sunday on a Palau-flagged dry cargo ship in the southwestern Black Sea as it made its way towards Ukraine, the Russian defense ministry said.

Russia last month scuppered a U.N.-brokered grain deal that ensured Ukraine could get its agricultural produce to market via the Black Sea and Moscow cautioned that it would deem all ships heading to Ukrainian waters to be potentially carrying weapons.

Russia said in a statement that its Vasily Bykov patrol ship had fired automatic weapons on the Sukru Okan vessel after the latter’s captain failed to respond to a request to halt for an inspection.

The Sukru Okan was making its way towards the Ukrainian port of Izmail, the defense ministry said. Refinitiv shipping data showed the ship was heading north towards the coast of Bulgaria.

“To forcibly stop the vessel, warning fire was opened from automatic weapons,” the Russian defence ministry said.

The Russian military boarded the vessel with the help of a Ka-29 helicopter.

“After the inspection group completed its work on board, the Sukru Okan continued on its way to the port of Izmail,” the defense ministry said.

Reuters could not immediately reach the vessel or its owners for comment.

Ukraine and the West say Russia’s steps amount to a de-facto blockade of Ukrainian ports that threatens to cut off the flow of wheat and sunflower seeds from Ukraine to world markets.

Ukraine’s response — sea-drone attacks on a Russian oil tanker and a warship at its Novorossiysk naval base, next door to a major grain and oil port — has added to these new dangers for transport in the Black Sea.

Russia and Ukraine are two of the world’s top agricultural producers, and major players in the wheat, barley, maize, rapeseed, rapeseed oil, sunflower seed and sunflower oil markets. Russia is also dominant in the fertiliser market.