Ukraine's navy says Russia has fired on and seized three of its vessels off the coast of the annexed Crimea, in a major escalation of tensions.
It said two gunboats and a tug were captured by Russian special forces and six crew members were injured.
Ukraine said would meet to declare martial law over the incident.
The crisis began when Russia accused the Ukrainian ships of illegally entering its waters and stopped them from sailing into the Sea of Azov.
The Russians placed a cargo ship under a bridge in the Kerch Strait. It is the only access to the Sea of Azov, which is shared between the two countries.