The Financial Times newspaper had first reported that the Heng Yang 9, owned by the Guangxi Changhai Shipping Company, has docked in Sevastopol at least thrice this year, citing satellite data.

The Ukrainian foreign ministry strongly condemned the visits, calling them "a gross violation of international law and Ukrainian legislation."

The Panama-flagged ship has "deliberately concealed its movements by transmitting false data about its route," the ministry added.

Western sanctions, imposed almost a decade ago when Russia annexed Crimea, bar ships from docking in the Black Sea peninsula's ports.

China, the biggest buyer of Russian fossil fuels, has not joined the Western-imposed sanctions but has advised citizens to avoid doing business with sanctioned entities or on occupied territories in Ukraine.

Ukraine said it had sent an official note to the Chinese foreign ministry and had informed the International Maritime Organization.

According to FT, the Chinese foreign ministry said it will look into the incidents, while confirming the guidance for citizens.