Russia was behind Tuesday's drone attack that sank a merchant ship in Bulgaria's exclusive economic zone and left its crew missing, Ukrainian president Volodymyr Zelensky said. A second ship was badly damaged and caught fire.

This report follows our earlier story on the strikes on two ships in Bulgaria's economic zone, in which one vessel sank and the other caught fire.

The ships were hit about 70 nautical miles from Byala at almost the same time, while sailing in opposite directions. One had arrived in the area shortly before, having left Ukraine the day before. Bulgaria is a NATO member.

Later on Tuesday, Zelensky said Moscow was preparing a bigger "massive strike". He said Ukrainian intelligence and security services were working to establish the circumstances of the attack.

Cameron Chell, founder of drone company Draganfly, believes the first strike probably used an aerial Geran-4 drone or a Maran naval drone. He does not think the ship was hit from the air alone. Geran drones attack in their final phase at a steep angle, and to sink a ship quickly the hit has to come low, near the water, Chell said.

Few precision drones can fly low over the surface, so he believes the attack combined air and sea weapons. He said the coordination stood out: the attackers used advanced technology and timed the strikes almost together, which Chell called a new level.

He believes the two precise strikes are a dangerous escalation. In his view, Russia may be preparing a larger incursion and wants to raise the stakes.

Chell also says the hybrid nature of the attack blurs the line with conventional warfare and is an "expansion of strategy and tactics". He adds that the place and manner of the strike could help Russia increase uncertainty in the region.