Greek Defense Minister Nikos Dendias, speaking at a meeting of European Union defense ministers in Brussels on Tuesday, confirmed that a military sea drone discovered last week on the Ionian Sea island of Lefkada is a Ukrainian unmanned surface vehicle. He called the finding “an extremely serious issue” and said it endangered the freedom and security of navigation in the Mediterranean.

“We have certainty now that it is a Ukrainian USV,” Dendias said, using the abbreviation for an unmanned surface vessel. He added that he would raise the matter directly with Ukrainian officials.

The drone, which was carrying explosives, was found by a local fisherman inside a coastal cave on May 7. The fisherman towed it to a harbor, and on May 8 Greek authorities moved it to a naval base on the mainland for inspection. The explosives were later destroyed, the state broadcaster ERT reported.

“It appears that the [drone] suffered some malfunction and was moving in an uncontrolled way,” Stefanos Gikas, a Greek deputy minister for maritime affairs, told public television on Monday. “So this craft — a black thing without navigation and carrying explosives — could have struck a tourist vessel.” Lefkada lies along a busy shipping lane between Greece and Italy, popular with yachts, ferries and commercial ships.

Ukraine has deployed sea drones extensively against Russian warships and oil tankers in the Black Sea, and the incident underscores how the conflict’s drone warfare is increasingly spilling into the territory of NATO and European Union member states. At the same EU meeting, Romanian Defense Minister Radu-Dinel Miruța described a pattern of airspace violations by suspected Russian drones.

“They are violating our airspace. And it’s very clear that inside the European Union we should rearrange our capacities, our capabilities, in order to decrease this type of violations,” Miruța said. “It is very important to understand that this is a common threat. It is happening on the entire eastern flank.”

Ukrainian authorities did not immediately respond to a request for comment. Greek naval experts said the recovered craft’s features resemble Ukraine’s Magura-type surface drones, a platform developed by the country’s intelligence service.