In the unprecedented incident, Russia said it used weapons "in order to stop the Ukrainian military", which it claims illegally entered its waters, confirming "three Ukrainian navy ships were boarded and searched".

Ukraine's navy said the incident took place on Sunday as two small warships and a tugboat were heading through the Kerch Strait, a narrow waterway that gives access to the Sea of Azov that is used by Ukraine and Russia.

It said a Russian border guard vessel rammed the tugboat in "openly aggressive actions" and then fired on the ships, immobilising all three.

CNN reports Ukraine and Russia offered conflicting accounts of the incident, each accusing the other side of violating the laws of the sea. A 2003 agreement confirms the Azov Sea and the Kerch Strait as domestic waters of Russia and Ukraine.

Russia's Federal Security Service (FSB) Border Service in Crimea reported that three Ukrainian warships had illegally entered Russia's territorial waters, and were carrying out dangerous manoeuvres, according to Russian state news agency TASS.

TASS reported the three Ukrainian vessels were detained by Russian forces and that "weapons were used to force them to stop".

"They did not respond to legitimate demands by the ships and boats of Russia's FSB Border Guard Service escorting them to stop immediately and performed dangerous manoeuvres," the FSB said, according to TASS.

However, Ukraine said it had informed the Russians in advance of its plan to move its ships through the sea to Mariupol, and said their seizure was "another act of armed aggression" by Russia.

News24/CNN