A Russian vessel shot warning rounds at a cargo ship heading north in the Black Sea on Sunday, the very first time Russia opened fire on merchant vessels outside Ukraine since leaving a significant UN-brokered grain deal last month.

In July, Russia suspended involvement in the Black Sea grain arrangement, enabling Ukraine to export agricultural products across the Black Sea, and warned that all ships heading to Ukrainian seas could be carrying weapons.

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

Russia said the vessel was making its way toward the Ukrainian port of Izmail. Refinitiv shipping data showed the ship was currently near the coast of Bulgaria and heading towards the Romanian port of Sulina.

“Warning fire from automatic weapons was opened to forcibly stop the vessel,” the Russian defense ministry claimed.

According to the government, the Russian military boarded the vessel using a Ka-29 chopper.” After the inspection group completed its work on board, the Sukru Okan continued its way to the port of Izmail,” according to the defense ministry. An official from Turkey’s defense ministry said he had heard of an incident involving a ship bound for Romania and that Ankara was looking into it.

Reuters was unable to reach the yacht or its owners for comment immediately. Ukraine did not respond promptly to the tragedy.

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 fertilizer market. Since Russia left the Black Sea grain deal, both Moscow and Kyiv have issued warnings and carried out attacks that have sent jitters through global commodity, oil, and shipping markets.

Russia has said it will treat any ships approaching Ukrainian ports as potential military vessels, and their flag countries as combatants on the Ukrainian side. Russia also struck Ukrainian grain facilities on the Danube.

Ukraine responded with a similar threat to ships approaching Russian or Russian-held Ukrainian ports. Ukraine also attacked a Russian oil tanker and a warship at its Novorossiysk naval base, next door to a major grain and oil port. 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.

Russia dismisses that interpretation and says the West failed to implement a parallel agreement easing rules for its own food and fertilizer exports.