russian subs
Russian Black Sea Fleet submarines are seen in the Crimean port of SevastopolReuters

Nato sent a number of patrol planes to help the UK search the waters off the western Scottish coast after a submarine's periscope was spotted.

The UK's Ministry of Defence (MoD) said it had been helped by Nato allies in an operation, although it did not give further details.

As many as five search planes from four countries worked with Royal Navy ships to try and locate the submarine.

Searches took place in November and December, after Sweden reported a similar occurrence in its territorial waters in October. The Swedes never found the elusive vessel.

A squadron of Russian warships entered the English Channel in November amid rising tensions between Moscow and the West over the Ukraine crisis.

The European Union, United States and other allies have accused Russia of fuelling the conflict in eastern Ukraine and imposed wide-ranging sanctions on Moscow. For its part, Russia has denied involvement in the fighting and imposed a ban on Western food imports.

"Nato partners have provided assistance for the operation of maritime patrol aircraft from RAF Lossiemouth for a limited period with the Royal Navy. We do not discuss the detail of maritime operations," the UK's MoD said in a statement.

The submarine's periscope was spotted in waters usually frequented by British vessels as they enter or leave the Royal Navy's Faslane submarine base.