North Korean leader Kim Jong Un called for the country to stand ready to conduct nuclear attacks at any time to deter war, accusing the US and South Korea of expanding joint military drills involving American nuclear assets, state media KCNA said on Monday.

Kim's remarks came as the isolated country conducted what KCNA called exercises aimed at bolstering its "war deterrence and nuclear counterattack capability" on Saturday and Sunday to send strong warnings against the allies.

In the exercises, a ballistic missile equipped with a mock nuclear warhead flew 800 km (497 miles) before hitting a target at the altitude of 800m under the scenario of a tactical nuclear attack, KCNA said.

Kim, who oversaw the test, said the exercises improved the military's actual war capability and highlighted the need to ensure its readiness posture for any "immediate and overwhelming nuclear counterattack" through such drills.

"The present situation, in which the enemies are getting ever more pronounced in their moves for aggression against the DPRK, urgently requires the DPRK to bolster up its nuclear war deterrence exponentially," KCNA quoted him as saying.

Kim was using the acronym of his country's official name, the Democratic People's Republic of Korea.

"The nuclear force of the DPRK will strongly deter, control and manage the enemy's reckless moves and provocations with its high war readiness, and carry out its important mission without hesitation in case of any unwanted situation," he added.

KCNA photos showed Kim attended the test, again with his young daughter, as flames roar from the soaring missile before it hit the target.

South Korea and Japan reported a launch of a North Korean short-range ballistic missile off the east coast on Sunday, the latest in a series of missile tests in recent weeks.

North Korea has reacted furiously to South Korea-US combined military drills, calling them a rehearsal for invasion against it.

The allies have been carrying out a multitude of their annual exercises since earlier this month, including air and sea drills on Sunday involving US B-1B strategic bombers.

The US and South Korea navies and marine corps are set to kick off their first large-scale Ssangyong amphibious landing exercises in five years on Monday for a two-week run until April 3.

Last month, the two countries staged tabletop exercises simulating North Korea's nuclear attack amid South Korean President Yoon Suk Yeol's push for more confidence in US extended deterrence - its military capability, especially nuclear forces, to deter attacks on its allies.

In another dispatch, KCNA said more than 1.4 million North Koreans have volunteered to join or re-enlist in the military to fight against Seoul and Washington, up from some 800,000 reported by a state newspaper just two days before.