Two British warships are shadowing an aircraft carrier and other Russian naval ships as they pass the UK on their way to Syria.
Sources indicate the carrier Admiral Kuznetsov and its task force may sail through the English Channel overnight, after leaving the North Sea.
A Ministry of Defence spokesman said the ships would be “man-marked every step of the way” while near UK waters.
However, Nato told Russia had the right to operate in international waters.
The Russian task force's journey comes amid heightened tension between Russia and Nato.
“We will be watching as part of our steadfast commitment to keep Britain safe, ” Defence Secretary Michael Fallon said.
The Ministry of Defence told at about 13:00 BST the task force was “in the middle of the North Sea heading southwards”.
At that stage it was understood to be about 100 miles( 160 km) off Edinburgh, but the MoD has not any further updates.
Type 45 destroyer HMS Duncan, escorted by the Type 23 frigate HMS Richmond, sailed from Portsmouth on Tuesday to track the Kuznetsov group as it headed south from the Norwegian Sea,