According to the preliminary findings of experts, the explosion of a vessel of the Ukrainian Border Service near Mariupol on Sunday was caused by an improvised explosive device (IED), the press center of the anti-terrorist operation (ATO) has reported.

"During the examination of the scene and the damaged boat, experts came to a conclusion that an improvised explosive device went off there," the ATO press center said in a statement on its Facebook page on Monday morning.

Boarder guard crewmen saw an unknown object in the sea, mistook it for a beacon buoy and tried to pull it towards the ship. At this point, the device went off, damaging the boat and injuring those onboard, the press center said.

"It should be noted that this is not the first such incident. Recently, a similar device was cast ashore near the village of Vynohradne. A fisherman who found it, was trying to take it apart, and this attempt ended tragically for him," the ATO press center said.

What both cases have in common is that the explosive devices were drifting from territory under the control of militants, in particular from the direction of the village of Bezimenne, as in both cases there was a strong wind from the east.

The Ukrainian State Border Service previously reported that a coastguard vessel of the Mariupol coastguard detachment, which had seven people onboard, exploded in the afternoon on Sunday for an unidentified reason while it was leaving the base. One person was killed, the ship's captain is still missing and five others are being treated in hospital.