Seoul's military said Thursday's tests, which involved "various types" of short-range ballistic missiles, could be linked to the nuclear-armed North's weapons exports to Moscow.

The drills involved a 600-millimetre multilayer rocket system and a tactical ballistic missile, Pyongyang's official Korean Central News Agency said.

Detailed trainings on "procedures and processes for rapidly switching over to the nuclear counterattack posture" took place, according to KCNA.

"The goal of the drill was achieved and the reliability of the command and mobilisation system capable of quickly reacting to any nuclear crisis was verified," the news agency added.

Kim earlier this week inspected shell manufacturing at munitions factories, with state media claiming the country's shell output had increased dramatically in recent months.

Pyongyang also recently unveiled a 5,000-ton destroyer-class vessel named Choe Hyon, that analysts say could carry both ship-to-surface and ship-to-air missiles.

Seoul's military said the warship could have involved Russian help, such as technology or funds, possibly in exchange for sending troops to help Moscow in its war in Ukraine.

North Korea confirmed last week for the first time it had deployed troops to Russia to support Moscow in the Ukraine war.

Pyongyang and Moscow also recently announced that they had started building the first road bridge linking the two neighbours.

Washington -- Seoul's key security ally -- has in recent years ramped up joint military exercises and increased the presence of strategic US assets, such as an aircraft carrier and a nuclear-powered submarine, in the region to deter the North.

South Korea's military said in early March that the North fired "multiple unidentified ballistic missiles", the same day Seoul and Washington began a major annual joint military drill known as Freedom Shield.

Pyongyang has repeatedly declared itself an "irreversible" nuclear weapons state and routinely denounces joint US-South Korea drills as rehearsals for invasion.