Moscow said Sunday that warning shots were fired from a Russian warship at a cargo vessel heading towards the Ukrainian port of Izmail.
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.
Russian forces kill six including baby in southern Ukraine: official
Six people, including a baby girl and her parents, were killed on Sunday by Russian shelling in southern Ukraine, officials said.
"Russia killed six people in the region of Kherson," Interior Minister Igor Klymenko said on messaging app Telegram, releasing pictures showing the aftermath of the attack.
One photo showed a column of black smoke rising above a house.
Three adults and a small baby were killed in the village of Shyroka Balka.
"A husband, wife and their 23-day-old daughter were killed by enemy artillery fire," Klymenko wrote.
"Their 12-year-old son was hospitalized in serious condition. The child's condition is critical."
In the village of Stanislav two men were killed, and another person received injuries.
"Terrorists must be stopped. They must be stopped by force," said Klymenko.
Kherson was one of four regions in Ukraine that Russian President Vladimir Putin claimed to have annexed in September.
Late last year, Russian troops withdrew from the city of Kherson but has continued to target settlements in the region.