Pyongyang, North Korea - North Korea fired another round of cruise missiles on Tuesday, according to Seoul's military, extending a week-long flurry of tests that Pyongyang has said include a new generation of weapons.

South Korea's military "detected several unknown cruise missiles launched into the West Sea of North Korea around 07:00 (5 PM ET Monday)", Seoul's Joint Chiefs of Staff said in a statement.

South Korean and US intelligence agencies "are conducting a detailed analysis", the JCS said.

"Our military is cooperating closely with the U.S. while strengthening surveillance and vigilance, and is closely monitoring North Korea's activities," it added.

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Unlike their ballistic counterparts, the testing of cruise missiles is not banned under current UN sanctions on Pyongyang, but the country has accelerated weapons testing in the new year, including tests of what it called an "underwater nuclear weapon system" and a solid-fuelled hypersonic ballistic missile.

On Monday, state media said North Korean leader Kim Jong-un had overseen the test launch of a new strategic cruise missile from a submarine.

Last week, Pyongyang said it had also conducted its first test of a new generation of strategic cruise missiles.

Recent months have seen a sharp deterioration in ties between the two Koreas, with both sides jettisoning key tension-reducing agreements, ramping up frontier security, and conducting live-fire drills along the border.

Kim has declared the South a "principal enemy," closed agencies dedicated to reunification and outreach, and threatened war over "even 0.001 millimeters" of territorial infringement.

He also said Pyongyang would not recognize the two countries' de facto maritime border, the Northern Limit Line, and called for constitutional changes allowing the North to "occupy" Seoul in war, the North's official Korean Central News Agency said.

Pyongyang's latest launch comes after South Korea conducted a 10-day special forces infiltration drill off the country's east coast, "in light of serious security situations" with the North, which ended on January 25.