The report comes a day after North Korea fired multiple short-range ballistic missiles into waters east of the Korean peninsula, Seoul's military said, the nuclear-armed country's first major weapons test since early July.

Kim inspected the Nuclear Weapons Institute and a production base of weapon-grade nuclear materials, the government news agency KCNA reported.

As he toured the facility, Kim "encouraged the nuclear scientists faithful to the Party to unconditionally implement the Party's policy of building up nuclear force without an inch of deflection with their strong faith and high practical ability," KCNA said.

Noting that nuclear threats from the United States and its allies "have become more undisguised and crossed the red line," Kim said North Korea must "steadily expand and bolster up its defence capability... and the capability for a preemptive attack with the nuclear force as a pivot."

Kim's regime has staged dozens of launches this year, part of a testing spree experts say could be linked to North Korea's alleged illicit supplying of weapons to ally Russia for use in Ukraine.

Pyongyang has denied any sanctions-busting weapons trade with Russia, but with diplomacy long stalled, it declared South Korea its "principal enemy" this year and recently moved nuclear-capable weapons to border areas.

Seoul's Joint Chief of Staff said it had detected multiple "short-range ballistic missiles" fired early Thursday from Pyongyang into the East Sea, or Sea of Japan.

The missiles splashed down after flying around 360 kilometres (220 miles), the JCS said, adding it had "immediately detected, tracked, and monitored" the launch and was sharing information with allies Tokyo and Washington.

It said the test was "a clear provocation that seriously threatens peace and stability on the Korean Peninsula".

Japan's defence ministry also confirmed the missile test.

In an apparent reference to that test, KCNA reported Friday that Kim "oversaw the test-fire for verifying the performance of a new-type 600mm multiple rocket launcher."

The shells "hit the target on an island in the East Sea," according to KCNA.

"The test was conducted, aiming at further developing the launcher's driving system and verifying the combat effectiveness of the launcher in which the whole process of firepower is fully automated," it added.