Russia's main intelligence agency said several hundred mines had drifted into the Black Sea after breaking off from cables near Ukrainian ports, a claim dismissed by Ukraine which said it was disinformation and an attempt to close off parts of the sea.

The Black Sea is a major shipping artery for grain, oil and oil products. Novorossiisk Port Authority, in a note seen by Reuters, said that shipping was at risk in the western Black Sea.

"Due to storm weather, the cables connecting the mines to anchors were broken," Russia's Federal Security Service (FSB), the main successor to the Soviet-era KGB, said in a press release dated March 19.

"Due to wind and water currents, the mines are drifting freely in the western part of the Black Sea," the FSB said.

The FSB said about 420 mines had broken loose, claiming that the mines were set by Ukrainian forces. Ukraine said the FSB's warning was wrong and that it had no information about any mines drifting out to sea.

"This is complete disinformation from the Russian side," Viktor Vyshnov, deputy head of Ukraine’s state-run Maritime Administration, told Reuters.

"This was done to justify the closure of these districts of the Black Sea under so called 'danger of mines'"

The Ukrainian navy said it alone had the right to distribute safety notices about its part of the Black Sea and cast Moscow's warning as an attempt at information "piracy".

Russia's Feb. 24 invasion of Ukraine has killed thousands of people, displaced more than 3 million and raised fears of a wider confrontation between Russia and the United States.

President Vladimir Putin says the "special military operation" in Ukraine is necessary because the United States was using Ukraine to threaten Russia and Russia had to defend against the "genocide" of Russian-speaking people by Ukraine.

Ukraine says it is fighting for its existence against a Russian imperial-style land grab and that Putin's claims of genocide are nonsense.

The West has imposed sweeping sanctions on Russia that the Kremlin says amount to a declaration of economic war by the United States and its allies. China has called for calm.

Shipping sources said Turkish authorities had cautioned about the risk of mines for those using the Dardanelles and Bosphorus straits.

Turkey's Samsun NAVTEX station issued a warning about mines in the approaches to Odesa and other Ukrainian ports.

"It is important to keep sharp look out for drifted mines by all ships in the vicinity," it said. "It is requested from ships in the vicinity to report immediately any detection of drifted mine or mine like object." (Editing by Guy Faulconbridge and Philippa Fletcher)