ISTANBUL (Reuters) - Turkish authorites have warned vessels to stay away as a dive team inspects a "mine-like object" found floating north of Istanbul near the Black Sea, days after Russia warned that several mines had drifted away from Ukrainian ports.

       The team secured the object and began deactivating it, Turkey's defence ministry said on Twitter.

       Fishermen first spotted the object near a docking area in the upper Bosphorus strait - through which the Black Sea drains - and reported it to the Coast Guard, the Directorate General of Coastal Safety told Reuters.

       A directorate spokesperson confirmed a report in the Milliyet newspaper that authorities issued a radio notice to boats that the object was reportedly "shaped like a ball with horn-like protrusions", and was "likely to be a mine".

       Turkey shares Black Sea borders with Russia and Ukraine, which Moscow invaded last month.

       Russia's main intelligence agency said on Monday that several mines had drifted out to sea after breaking off from cables near Ukrainian ports, a claim dismissed by Kyiv as 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.

       (Reporting by Azra Ceylan; Writing by Jonathan Spicer; Editing by Barbara Lewis)