A Romanian military dive team was trying to defuse a mine detected in the Black Sea by a fishing boat on Monday about 70 kilometers offshore.
It was one of several found drifting in recent days.
The Romanian Navy sent a minesweeper, the Vice Admiral Constantin Bălescu, to investigate and eliminate potential danger.
Earlier, Turkish authorities secured a mine close to the border with Bulgaria. On Saturday, they deactivated another ordinance which set off a loud blast north of Istanbul.
Russian intelligence officials said earlier in March that several mines had drifted to sea after breaking off from cables near Ukrainian ports. The Ukrainian government dismissed the claims calling them disinformation and an attempt to close off parts of the sea.
Romania has a Black Sea border, with Bulgaria, Turkey and Ukraine which Russia invaded a month ago. The sea is a major shipping route for grain and oil.
Fishermen first spotted the object earlier on Monday and reported it to the Romanian naval forces.