A British frigate shadowed a Russian warship through the North Sea near UK waters on Christmas Day, the Royal Navy has revealed.
HMS St Albans monitored the Admiral Gorsdhkov's "activity in areas of national interest", it said.
The Admiral Gorshkov, a new guided-missile frigate, is still undergoing trials, Russian media report.
The Royal Navy says there has been a recent "upsurge in Russian units transiting UK waters".
How was the Russian ship tracked?
HMS St Albans was sent on Saturday to "keep watch on the new Russian warship Admiral Gorshkov as it passed close to UK territorial waters", the Royal Navy said.
The British frigate remained at sea on Monday, monitoring the Russian vessel, and was due to return to Portsmouth on Tuesday.
"I will not hesitate in defending our waters or tolerate any form of aggression," Defence Secretary Gavin Williamson said.
"Britain will never be intimidated when it comes to protecting our country, our people, and our national interests."
In other recent activity involving the two navies:
- HMS Tyne, a patrol ship, was also called to shadow a Russian intelligence-gathering ship as it sailed through the North Sea and the English Channel on Sunday while a navy helicopter was scrambled to track two other Russian vessels
Why was the Russian ship in the North Sea?
The Admiral Gorshkov, the first of a new class of multi-role frigates, has still to complete missile tests before entering service with the Russian navy next year, Russian media report.
It has reportedly been sailing regularly between the White Sea off Russia's northern coast and the Baltic.
Rear Admiral Chris Parry, a former Royal Navy Officer and former Nato commander, describes the deployment of the war ship as "normal".
He told the BBC: "She's perfectly entitled to do that under international law. It's demonstrating the right of innocent passage."
Reports on the latest interception do not make clear in what direction the Russian ship was heading.
Russian warships have used the international waters of the North Sea in recent times to sail to and from the Mediterranean for deployment off Syria.
Why is the UK worried about undersea cables?
Relations between Britain and Russia have remained tense since Moscow's annexation of Ukraine's Crimea peninsula in 2014.
Air Chief Marshal Sir Stuart Peach, the chief of the UK's defence staff, said earlier this month that Britain and Nato needed to prioritise protecting the lines of communication.
He said it would "immediately and potentially catastrophically" hit the economy if they were cut or disrupted.
The cables criss-cross the seabed, connecting up countries and continents.