Russia has seized three Ukrainian naval ships off Crimea
Russia says it opened fire to force the Ukrainian ships to stop
Between three and six people were injured in the incident
The Kremlin accused Ukraine of illegally entering its waters
Ukraine's parliament may vote on declaring martial law on Monday
Ukraine's national security council says President Petro Poroshenko has called an urgent meeting of his military cabinet to discuss the crisis.
Ukraine's foreign ministry says "these aggressive actions" are a violation of international law and will be met with "an international diplomatic and legal response".
Our Kiev correspondent Jonah Fisher has been monitoring the situation.
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Tensions have been rising in the Sea of Avov, which lies south of the Ukrainian regions partially seized by pro-Russian separatists.
A 2003 treaty guarantees free navigation to Russian and Ukrainian vessels, but Russia has recently increased its military presence.
It has begun inspecting all vessels going to or from Ukrainian ports, with some calling Moscow's actions an "economic blockade".
We're bringing up you updates on a major escalation of tensions between Russia and Ukraine.
The Ukrainian navy has reported that three of its vessels were seized by Russia off the coast of annexed Crimea.
Ukraine said Russian special forces chased and captured two gunboats and a tug and two crew members were injured in the incident.
Russia has yet to comment on the claims, but it earlier accused Ukraine of illegally entering Russian waters in the Sea of Azov.