A Romanian military dive team was trying to defuse a mine detected some 70 kilometres offshore in the Black Sea, the defence ministry said on Monday, one of several found drifting in recent days.
Earlier on Monday, Turkish authorities secured a mine close to the border with Bulgaria. On Saturday, they deactivated another ordinance, setting off a loud explosion north of Istanbul.
Russia's main intelligence agency said earlier in March that several mines had drifted 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.
Romania shares borders of the Black Sea - a major shipping artery for grain and oil - with Bulgaria, Turkey and Ukraine, which Russia invaded a month ago by land, air and sea.
Fishermen first spotted the object earlier on Monday and reported it to the Romanian naval forces.
(Reporting by Luiza Ilie; Editing by Bernadette Baum)