The agreement between the two NATO allies aims to protect critical undersea infrastructure, such as cables, that Western officials say is increasingly under threat from Moscow.

Britain's Ministry of Defence (MoD) has reported that sightings of Russian vessels in UK waters have increased 30 percent in the past two years.

Norwegian Prime Minister Jonas Gahr Store was met by his UK counterpart Keir Starmer for talks at Downing Street, in London.

Store hailed "a very important agreement on defence cooperation and integration".

"This is really about the present. This is about acknowledging where Europe stands and what we need to take care of security for the future," he added.

The two countries were "making significant steps... because we share waters, we share the strategic environment".

Under the new pact, the two countries will operate a fleet of at least 13 British-built frigates on an "interchangeable" basis.

The fleet "will hunt Russian submarines and protect critical infrastructure in the North Atlantic," the defence ministry said in a statement.

It will monitor Russian naval activity in the waters between Greenland, Iceland and the UK, "defending critical infrastructure such as underwater cables and pipelines, which carry vital communications, electricity and gas", the MoD said.

"At this time of profound global instability, as more Russian ships are being detected in our waters, we must work with international partners to protect our national security," said Starmer.

- 'Safe into the future' -

Last month, UK Defence Minister John Healey warned Russia after saying that its military ship Yantar had entered British waters for the second time this year.

He said it had directed lasers at British air force pilots in a "deeply dangerous" move.

Britain and NATO allies have expressed growing concern about the risk Moscow poses to offshore infrastructure following the suspected sabotage in recent months of several undersea telecom and power cables.

Experts and politicians have accused Moscow of orchestrating a hybrid war against Western countries, most of which support Ukraine following Russia's full-scale invasion of the country in 2022.

Norway announced in September the purchase of at least five Type-26 frigates from Britain for pound10 billion ($13 billion).

BAE Systems beat out competing bids for the frigates from French, German and US groups.

Starmer and Store visited a Royal Air Force base in Lossiemouth, northern Scotland, later Thursday to mark the signing of the pact.

The deal also sets out plans for year-round training for Royal Marines in Norway, "preparing them to fight in sub-zero conditions," the MoD said.

And the UK will join a Norwegian programme "to develop motherships for uncrewed mine hunting and undersea warfare systems".

"We will patrol the North Atlantic as one, train together in the Arctic, and develop the advanced equipment that will keep our citizens safe now and into the future," said Healey.