Pyongyang, North Korea - North Korea fired several cruise missiles towards the Yellow Sea on Wednesday, Seoul's military said, the latest in a series of tension-raising moves by the nuclear-armed state.

Pyongyang 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.

"Our military detected several cruise missiles launched by North Korea towards the Yellow Sea at around 7:00 AM [local time] today," the Joint Chiefs of Staff said in a statement.

"The detailed specifications are being closely analyzed by South Korean and US intelligence authorities," it added.

Unlike their ballistic counterparts, the testing of cruise missiles is not banned under current UN sanctions against 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.

The latest launch comes as South Korea is conducting a 10-day special forces infiltration drill off its east coast, "in light of serious security situations" with the North, that runs until Thursday, according to the South's navy.

"We will achieve our mission to infiltrate deep into the enemy's territory and neutralize them completely under any circumstances," the drill's commander said in a statement.

Last week, North Korean leader Kim Jong-un declared the South his country's "principal enemy," shut down agencies dedicated to reunification and outreach, and threatened war would follow any kind of territorial infringement.

The North Korean leader 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 Korean Central News Agency (KCNA) said.

In Seoul, President Yoon Suk Yeol told his cabinet that should the nuclear-armed North carry out a provocation, South Korea would hit back with a response "multiple times stronger", pointing to his military's "overwhelming response capabilities".