NATO allies sent patrol planes to help Britain scour the waters off its western Scottish coast after a submarine's periscope was spotted, in a search reminiscent of the Cold War, media reported.
Britain's Ministry of Defense (MoD) said it had received help from NATO allies in an operation, but declined to go into further details.
Reports that searches took place in late November and early December, which first appeared in Aviation Week, come amid growing tensions between Western powers and Russia over its role in the Ukraine crisis.
A squadron of Russian warships entered the English Channel last month and Sweden said it had proof a foreign submarine was operating illegally in its waters in October.
A periscope was spotted in waters where Royal Navy submarines normally surface as they head in or out of a base at Faslane, home to Britain's nuclear deterrent, Aviation Week reported.
It said planes from France, the United States and Canada were involved in the search.
“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 MoD said in a statement late on Tuesday.
Britain has not had its own specialist maritime patrol aircraft since scrapping its Nimrod program as part of a defense review in 2010.
The MoD did not confirm the planes were looking for a submarine or when the operation took place. Aviation Week said it was not clear whether the submarine being hunted had entered British territorial waters.
The Estonian defense forces said the country's foreign ministry had called in the Russian ambassador to protest after a Russian military plane entered Estonia's airspace on Tuesday.
Fighter jets from NATO's Baltic air policing mission carried out several intercepts of Russian military aircraft in international airspace over the Baltic sea and off the coast of Norway on both Monday and Sunday, a spokeswoman for the military alliance said.
More than 200 troops, stealth ships and helicopters searched Baltic waters off the Swedish capital Stockholm in October after reports of foreign “underwater activity.”
That operation reawakened memories of the final years of the Cold War when Sweden repeatedly hunted suspected Soviet submarines along its coast with depth charges.