The Russians have reinforced their air power in Crimea with the arrival in Belbeck air base of more than a dozen Sukhoi Su-27 and SU-30 fighter-bombers.

The Russians illegally seized Crimea from Ukraine in 2014 and currently hold it by force. The country's other main eastern ports, notably Mariupol, are blockaded and last month three Ukrainian naval vessels and their crews were seized in the Sea of Azov by the Russians, who called their deployment an "act of deliberate provocation".

The deployment of the aircraft co-incides with the arrival in Odessa of HMS Echo, a Royal Navy hydrological survey ship. Also visiting was the UK defence Minister Gavin Wiliamson, who said HMS Echo was there to remind the Russians the Black Sea did not belong to them.

Russian Foreign Minister Sergei Lavrov has alleged Ukraine was preparing "a provocation" near Crimea before the end of the year.

Williamson added that Britain had sent a Royal Navy ship there to show Kiev did not stand alone.

He was speaking as tensions in the area remain high, and he also met Stepan Poltorak, his Ukrainian counterpart.

Relations between Moscow and Kiev have been locked in a downwards spiral since pro-Western forces overthrew the pro-Russian President Viktor Yanukovich and replaced him with a pro-Western government. Russia then annexed Crimea from Ukraine in 2014 and backed pro-Russian separatists in eastern Ukraine.