European Commission President Ursula von der Leyen pledged €50 million ($56 million) on Tuesday to help Bulgaria and Romania expedite the launch of a Black Sea maritime security hub, following a string of deadly drone attacks on cargo ships.

At least nine cargo ships were hit by naval drones near NATO members Romania and Bulgaria last month, killing several seafarers and leaving others missing, raising concerns over how the escalating maritime war between Russia and Ukraine is expanding into a major challenge for NATO in the Black Sea.

Speaking from the Bulgarian port of Varna, von der Leyen said all sides have agreed to speed up the hub to restore freedom of navigation in the Black Sea. "It is important to have it now up and running and therefore we want to have clear timelines for its completion and we want to offer a dedicated funding of 50 million euros to this Black Sea maritime security hub," she said at a press conference alongside Bulgarian Prime Minister Rumen Radev.

Bulgaria and Romania plan to jointly set up the EU-mandated hub to improve the sharing of awareness and information and bolster security of a waterway that is key to trade, energy and food supplies from Ukraine to millions of people.

The U.S.-led military alliance said it would deploy maritime patrol aircraft, radars and drones to bolster its eastern flank in the Black Sea.

Bucharest has accused Russia of "systematic" strikes on civilian ships but stopped short of attributing individual attacks, citing the need for forensic evidence. Sofia has not blamed Russia, but said some of the attacks seem to benefit it. Moscow has denied involvement in the attacks.