A Russian warship fired warning shots and boarded a cargo ship it claims was headed to Ukraine in the Black Sea on Sunday, according to Russia’s Defense Ministry.

Russia pulled out of a UN and Turkish-brokered deal in July that allowed Ukraine to move its grain via the Black Sea and warned that any ships headed to Ukraine would be treated as potentially carrying weapons. Ukraine made a similar threat to ships traveling to Russian ports.

Russia said the warship fired warning shots when the captain of the Palau-flagged dry cargo ship failed to respond to a request to stop for an inspection.

“The Russian warship opened warning fire from automatic small arms fire to forcefully stop the vessel,” the statement said.

The ministry claimed the ship – named Sukra Okan – was headed to the Ukrainian port of Izmail. Marine traffic websites currently shows the cargo vessel’s destination as the Romanian port of Sulina which is close to Izmail. Kyiv did not immediately comment on whether or not the ship was headed to a Ukrainian port.

“In order to inspect the bulk cargo ship, a Ka-29 helicopter with a group of Russian servicemen was hoisted from the patrol ship Vasily Bykov,” the ministry said. “Following radio conversations, the ship stopped its course and the boarding team landed on the bulk cargo ship,” the statement said.

This week Ukraine announced that it would open up a temporary humanitarian corridor for ships to sail to and from its ports and has opened up registration for merchant vessels to use the sea route.

Both Russia and Ukraine are major grain producers and their deal – a rare point of agreement in the middle of a war – did much to stabilize prices.

Kyiv argues that Russia’s withdrawal amounts to a blockade of Ukrainian products. Russia long complained that it had been unable to export its own foodstuffs.

A Ukrainian Navy spokesperson, Dmytro Pletenchuk, said the temporary routes aim to overcome the global food security crisis and added that they would allow shipowners and companies to “finally take back their merchant vessels that are in humanitarian captivity due to the constant threats of Russians at sea.”

Pletenchuk said ship owners and captains have been warned of the existing danger and said Ukrainian Armed Forces will help to ensure the security of the merchant ships sailing through the corridors, with the Navy “doing everything we can to ensure security.”

It remains unclear when ships might use the route given the potential dangers there.