The pro-Russian authorities in Ukraine's Crimea region say a referendum will be held on 16 March on whether the peninsula should become part of Russia.

Ukraine's interim government has said any such vote would be unconstitutional.

The announcement comes as tension between the two sides remains high.

A military stand-off has been taking place in the Crimean port of Sevastopol with sailors from the Ukrainian navy being prevented from leaving their ships by Russian forces.

Christian Fraser gained rare access to the Ukrainian fleet stranded in the harbour.