The Ukrainian president's chief of staff said on Friday Russia was threatening civilian vessels in the Black Sea, and urged the international community to condemn what he said were "the methods of terrorists."
Russia last week quit a deal brokered by the United Nations and Turkey allowing Ukraine to safely export grain via the Black Sea and warned that ships heading to Ukrainian seaports could be considered military targets.
"Russian warships are threatening civilians in the Black Sea, violating all norms of international maritime law," Andriy Yermak, head of President Volodymyr Zelensky's office, wrote on the Telegram messaging app.
Warnings from a mystery Russian warship
In a separate statement, Ukraine's border guard service said it had intercepted a warning communicated by a Russian warship to a civilian vessel near a Ukrainian port on Thursday. It did not identify the name of the ship or the port.
The statement quoted the Russian party as saying: "I am warning you about the ban on movement to the ports of Ukraine."
"Also, the transport of any cargo to Ukraine is considered by the Russian side to be the potential transportation of military cargo," it was quoted as saying, adding that the country of the vessel's flag would be considered a party to the conflict in Ukraine.