United Kingdom Prime Minister Keir Starmer made a historic visit to a nuclear-armed submarine this week, becoming the first UK leader in more than a decade to board one of the nation's top-secret Vanguard-class vessels.

Starmer visited the submarine in Barrow-in-Furness, a shipbuilding town in northwest England, alongside John Healey, the UK's defense secretary.

Details of the prime minister's classified visit on Wednesday were kept secret until Downing Street released footage of it on Thursday.

The rare showcasing of the UK's nuclear deterrent after the vessel returned from a 200-day patrol on Wednesday coincided with international military planners meeting in London to discuss a peacekeeping force for Ukraine, timing likely to draw Russia's attention amid growing tensions, reported Reuters.

The visit preceded Starmer and Healey's tour of defense contractor BAE Systems' Barrow facility, where the next generation of nuclear submarines is under construction to replace the aging Vanguard fleet.

The unusual decision to release footage of a closely-guarded UK military asset appears intended as a message to Moscow, noted Sky News.

Speaking to news media after the tour, Starmer said Russia respected the UK's nuclear arsenal because "we've got our own independent deterrent and we're committed to NATO".

"What is obviously important is they appreciate that it is what it is, which is a credible capability," he said. "And that it most certainly is."

The PM highlighted Barrow-in-Furness as a model for how enhanced defense investment could drive economic growth across the UK.

"This week, I saw firsthand the sacrifice our submariners are making every day to keep our country safe, but I know they are only able to do that because of the support of the town of Barrow," he said.

"Each and every person living and working in Barrow is contributing to our nation's defense, whether that is building our world-class submarine program, or supporting the workforce here through vital public services or proud family businesses."

The visit highlighted ongoing challenges facing the UK's aging nuclear submarine fleet, which has been forced to extend its typical three-month patrols to much longer durations due to maintenance delays and the postponed delivery of replacement vessels, reported Sky. The current fleet has now exceeded its intended 25-year service life.

Military commanders from some 30 nations were set to convene on Thursday at a military base near London, advancing what Starmer recently described as the "operational phase" of plans for an international peacekeeping force in Ukraine.

Starmer, who is spearheading the initiative alongside France's President Emmanuel Macron, was due to address the gathering amid significant challenges, including demands from Russian President Vladimir Putin to halt Western military aid to Ukraine and Russia's opposition to any foreign troops on Ukrainian territory as conditions for a ceasefire.