Bulgaria's defence ministry has stepped up monitoring and control over the Black Sea, the ministry said. Air force planes are patrolling Bulgarian territorial waters after drone attacks on merchant ships in the sea.

Four types of aircraft are flying the missions: F-16, MiG-29, Su-25 and L-39. They patrol, observe and gather intelligence over Bulgarian waters. The flights will continue in the coming days. The ministry has also deployed extra ground forces, surveillance equipment and air defence systems to guard the airspace.

A Boeing C-17 Globemaster military transport plane landed at Burgas airport today. The landing coincided with the announcement of tighter monitoring, but the ministry confirmed the flight has no link to the security measures in the Black Sea.

The ministry says planes of this type have landed in Burgas before, so the arrival is not unusual. The aircraft often make technical stops, including to refuel. That was the case on 27 August, when a US Boeing C-17 landed in Burgas, refuelled and then took off.