"From 00:00 am Moscow time on 20 July 2023 (2100 GMT), all vessels on the Black Sea sailing to Ukrainian ports will be regarded as potential carriers of military cargo," the defence ministry said in a statement.

Kyiv has said it was prepared to continue grain exports through its southern ports, despite Russia pulling out of the deal that allowed safe passage through the Black Sea.

Ukrainian presidential aide Mykhailo Podolyak told AFP that Kyiv was considering a joint military patrol among Black Sea countries.

A day after the Kremlin warned of "risks" to establishing Black Sea routes without Moscow's participation, Russia issued the new, more concrete threat on Wednesday.

"The flag states of these vessels will be considered to be party to the Ukrainian conflict on the side of the Kyiv regime," it said.

It also said, "Some areas in northwestern and southeastern parts of the international waters of the Black Sea have been declared temporarily dangerous for passage."

Ships registered with a certain country -- their flag state -- must follow its regulations.