Since tensions erupted over Ukraine two years ago, Russian air and naval forces have had a number of close shaves with NATO
British warships on Thursday shadowed a Russian aircraft carrier battle group through the North Sea as NATO voiced fears the powerful force could soon join in attacks on Syria's besieged city of Aleppo.
NATO chief Jens Stoltenberg said the alliance would monitor the eight-ship force in "a responsible and measured" way as it headed to the Mediterranean, the latest cat-and-mouse encounter in two years of Cold-War style tensions.
"We are concerned Russia's carrier group will support military operations in Syria in ways which increase human and civilian suffering," Stoltenberg said at North Atlantic Treaty Organisation headquarters in Brussels.
"This group may be used to... increase attacks on Aleppo," the former Norwegian premier told a press conference after talks with Ukrainian President Petro Poroshenko.
Stoltenberg said NATO was also concerned about Russia's "continuing destabilisation" of eastern Ukraine, where Moscow-backed rebels are battling Ukrainian government forces two years after what he reiterated as Russia's illegal annexation of Crimea.
European Union leaders also meeting in the Belgian capital meanwhile warned Russia that they will consider sanctions over its role in the Syrian conflict if Moscow does not stop "crimes" in the devastated city of Aleppo.
NATO-Russia ties have sunk to lows not seen since the Cold War as President Vladimir Putin reasserts Russian power through intervention in both Ukraine and Syria.
A Royal Navy spokesman told AFP that the battle group spearheaded by the Admiral Kuznetsov aircraft carrier was the biggest deployment of Russian naval assets seen off British shores in recent years.
Britain's HMS Richmond frigate and the HMS Duncan air defence destroyer were monitoring the task force, which also included the nuclear-powered Pyotr Veliky battle cruiser and the Vice-Admiral Kulakov destroyer.
Defence Secretary Sir Michael Fallon said the Russian naval deployment was aimed at "testing" British and broader NATO capabilities
"The Russian fleet that is now sailing from the North ... is clearly designed to test the alliance. It's being marked every step of the way by the Royal Navy and ships and planes of other NATO members as well.
"It's clearly designed ... to test our response, and any weaknesses in the alliance, and we must make sure we respond in due measure," Fallon said.
The deployment comes after Russian Defence Minister Sergei Shoigu said the Admiral Kuznetsov, part of its Northern Fleet, would be sent to the eastern Mediterranean to boost its naval forces in the area.
Russia has been staging a bombing campaign in Syria for the past year in support of long-time ally President Bashar al-Assad and deployed a naval contingent to back up its operation.
A Royal Navy spokesman said the Russian ships were "behaving very well", including booking areas for flying and calling up coastguard stations.
A NATO official said separately the deployment "does not inspire confidence" that Moscow is seeking a political solution to the Syria crisis even as it implemented a temporary "humanitarian pause" to operations on Thursday.
Since tensions erupted over Ukraine two years ago, Russian air and naval forces have had a number of close shaves with their NATO counterparts.
In November, NATO member Turkey shot down a Russian warplane on the Turkish-Syrian border, sending relations into a tailspin before both sides recently mended fences.