SOFIA, Bulgaria (AFP) — A United States (US) military plane started to patrol the Black Sea following Sofia’s call for help after recent drone attacks on ships off the Bulgarian coast, Bulgaria’s prime minister said Tuesday.

NATO vowed to boost control over the Black Sea after drones struck several merchant ships off the Bulgarian and Romanian coast this month in the latest spillover of Russia’s invasion of Ukraine.

“An American P-8 Poseidon… has begun patrolling the Black Sea to conduct reconnaissance missions and detect any activity that could pose a risk,” said Prime Minister Rumen Radev.

He said some NATO countries were quick to answer his call for help after the attacks, and that Sofia expects “further assistance”.

A US Navy Boeing P-8A Poseidon arrived on Sunday in the Bulgarian port of Varna, having flown in from Sicily, according to data from air traffic tracking platform ADS-B Exchange.

Drones have hit four ships in attacks close to the Bulgarian coast this month, leaving 21 missing as two of the vessels sank.

Bulgaria summoned the Russian ambassador — as one of the hit ships was Ukrainian — and started investigating a war crime.

NATO chief Mark Rutte said in the wake of the attacks that the alliance would adjust its defences “as needed”.

NATO is set to discuss drone attacks targeting merchant vessels in the Black Sea during its meeting on Wednesday.