A Russian aircraft-carrier and other warships have reached waters near Trondheim in Norway and will soon head towards the UK.
The naval task force from Russia's Northern Fleet is expected to sail through the English Channel, then down to Gibraltar, and through the Mediterranean Sea to the Syrian coast.
The carrier Admiral Kuznetsov is escorted by seven other Russian ships.
The group will join the Russian combat mission in war-torn Syria.
It is the most powerful Russian naval task force to sail in northern Europe since 2014, Russia's Nezavisimaya Gazeta daily reports.
Nato has accused Russia of provocative military manoeuvres, especially in the Baltic region. Russian warplanes have stepped up patrols near Nato countries' airspace.
Lithuanian President Dalia Grybauskaite condemned Russia's recent deployment of nuclear-capable Iskander missiles to the Russian Baltic exclave of Kaliningrad.
Speaking in Helsinki on Tuesday, she called it an "aggressive, open demonstration of power" against not just the three Baltic states, but European capitals.
The aircraft-carrier is the only one in the Russian navy. It can carry more than 50 aircraft and its weapons systems include Granit anti-ship cruise missiles.
Next in the flotilla, in terms of firepower, is the Russian nuclear-powered battle cruiser Peter the Great.
A Norwegian Lockheed P-3 Orion reconnaissance plane, monitoring the force, photographed the ships. MiG-29 Fulcrum jets and combat helicopters were visible on the carrier's deck.
The other Russian surface ships in the group are: two large anti-submarine warships - the Severomorsk and Vice-Admiral Kulakov - and four support vessels.
"The Kuznetsov Task Group situation is normal routine for the Norwegian military," said Norwegian military spokesperson Maj Elisabeth Eikeland. "The only unusual thing is the amount of ships," she told the BBC.
The group will beef up the Russian naval presence off the Syrian coast - Russia already has about 10 ships there. During its bombardment of anti-government rebels in Syria that force has fired cruise missiles.
In an article headed "Moscow's Maritime Fist" the Russian armed forces channel TV Zvezda said several submarines would probably move from the Atlantic to escort the flotilla.
When the group reaches Syria the Russian navy will rival the firepower of the US Sixth Fleet in the region, Russian media report.
Commenting on the Russian flotilla, a Royal Navy spokesperson said: "UK and Nato assets routinely monitor warships from other nations when they enter our area of interest and this will be no different."