The FSB said that the vessel did not respond to demands to change course as it was moving towards the Kerch Strait in the Sea of Azov.
"Information about the movement of the control ship Donbass (A-500) of the Ukrainian Navy in the Azov Sea towards the Kerch Strait. The command ship Donbass lay on the opposite course from the Kerch Strait, does not respond to requests," the FSB said.
The command ship Donbass of the Ukrainian Navy left the roadstead of the port of Mariupol at 09:12 [06:12 GMT] and headed toward the strait. It was noted that at 13:45, an FSB accompanying coast guard ship warned the Ukrainian vessel of the need to comply with Russian legislation (in terms of filing the necessary applications for passage through the Kerch Strait).
An answer was received that it did not plan to pass through the Kerch Strait.
The FSB said that the Donbass crew's actions were regarded as provocative and endangered the safety of navigation.
The headquarters of the Joint Forces Operation in Donbas insisted that the ship did not enter the "sensitive zones" and performed tasks exclusively in its own territorial waters, adding that it was returning to the base.
Ukrainian Defense Minister Oleksii Reznikov also claimed that the Ukrainian ship was legally sailing in the Sea of Azov, since this was a shared water area.
"This is a search and rescue vessel without weapons, which was doing its job ... in the Sea of Azov, in neutral waters, in other words, was operating there legally ... I want to tell everybody that there was no threat to the crew and our vessel, she will soon return to the port, there were no incidents," Reznikov told the 1+1 broadcaster.
According to ship tracker MarineTraffic, more than 80 civilian ships were in the Kerch Strait at that time.