The Russian defence ministry said the Vasily Bykov patrol ship spotted a cargo vessel sailing under the flag of Palau "en route to the Ukrainian port of Izmail."

The captain of the Sukru Okan cargo ship did not respond to demands to stop for "the inspection for the transportation of prohibited goods."

"To force the ship to stop, warning shots from automatic small arms were fired from the Russian warship," the defence ministry said in a statement.

A helicopter carrying Russian military personnel was then scrambled to inspect the vessel, the statement said.

After the inspection was completed, the Sukru Okan was allowed to continue its journey.

Izmail is now the main export route for Ukrainian agricultural products.

Following its withdrawal from the Black Sea grain agreement in July, Moscow has pounded seaports in the southern Ukrainian region of Odesa that were key for the grain exports granted safe passage under the deal.