A Reuters witness saw the jets land at Belbek air base in Crimea, which was annexed by Russia in 2014 after Moscow-leaning Ukrainian President Viktor Yanukovich fled Kiev following street clashes and violent protests.

Tensions between Moscow and Kiev have risen in the past weeks after Russia seized three Ukrainian navy ships and their crews on Nov. 25 in an incident which Moscow and Kiev have blamed on each other.

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

UK ship in Ukraine 'sends message to Russia'

HMS Echo docked in the Black Sea on Friday, before Defence Secretary Gavin Williamson visited.

A Royal Navy warship which has been sent to Ukraine will send a strong message to Russian President Vladimir Putin, the UK's defence secretary says.

HMS Echo was sent into the Black Sea earlier this month, after Russia seized three ships belonging to Ukraine's navy and their crews.

Defence Secretary Gavin Williamson has visited the ship in the port of Odessa.

He said the presence of the British ship shows support for Ukraine in the face of increased Russian aggression.

It will be followed by other warships as part of a more constant British presence, he said.

Nato has recently stepped up its operations in the Black Sea with increased warship patrols.