Seoul, South Korea - North Korea fired multiple cruise missiles Friday, Seoul's military said, continuing a fresh streak of weapons testing as Kim Jong Un's regime ramps up what it calls "war preparations."

So far this year, Kim has declared South Korea his country's "principal enemy," jettisoned agencies dedicated to reunification and outreach, and threatened war over "even 0.001 mm" of territorial infringement.

Pyongyang has also carried out ever more weapons tests, including multiple cruise missile launches, an "underwater nuclear weapon system" test, plus firing a solid-fuelled hypersonic ballistic missile.

On Friday, Seoul's Joint Chiefs of Staff said the military had "detected at around 11 AM the firing of multiple unidentified cruise missiles" into the sea off the country's west coast.

The South Korean military has "stepped up surveillance in close coordination with the United States," the JCS said in the statement.

The military is "closely monitoring for signs of additional activity" by the North Korean army, it said, adding they were "closely analyzing" the launch.

Unlike their ballistic counterparts, the testing of cruise missiles is not banned under current UN sanctions on Pyongyang. Cruise missiles tend to be jet-propelled and fly at a lower altitude than more sophisticated ballistic missiles, making them harder to detect and intercept.

Analysts have warned that North Korea could be testing cruise missiles ahead of sending them to Russia for use in Ukraine, with Washington and Seoul claiming Kim has shipped weapons to Moscow, despite rafts of UN sanctions banning any such moves.

"North Korea could be using recent launches as a way to show Russians the capability of their missiles before sending them off to Moscow," Han Kwon-hee of the Korea Association of Defence Industry Studies.

"The need to do it could be especially acute given recent reports of North Korean shells going awry when used by Russian troops," he said.

Even as Kim ramps up the rhetorical threats against the South, the testing spree plus suspected Russian arms deals indicate "a dog that barks never bites," he said.

"If he was really serious about a war, he wouldn't have said it but kept it in the dark for a surprise attack. He also wouldn't have sold arms to Russia if he were really into going to war with the South."

The latest launch follows Kim Jong Un inspecting warships at a naval shipyard as he looks to bolster his maritime forces as part of "war preparations," state media said Friday.

"The strengthening of the naval force presents itself as the most important issue in reliably defending the maritime sovereignty of the country and stepping up the war preparations at present," Kim said at the Nampho Dockyard, according to the Korean Central News Agency.

A nuclear-powered submarine was on Kim's strategic weapons wish list set out at a key party congress in 2021, along with a hypersonic warhead, spy satellites, and solid-fuel intercontinental ballistic missiles.

During his visit to the dockyard, Kim was briefed on various warships under construction as well as preparations for a "new huge plan" assigned by the ruling party. No details of the plan were provided.

Kim "expressed expectation that the workers of the dockyard would successfully build on the world level the major warships," KCNA said.

North Korea last year launched what it called its first "tactical nuclear attack submarine," which Seoul's military at the time said did not look to be operational.

Analysts said the vessel appeared to be modified from an existing diesel-electric submarine originally designed in the 1950s, and have posed questions about its limitations and vulnerabilities as a platform.

The US stations about 30,000 troops in the South to help it fend off military threats from Pyongyang.