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 U.S. 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 improving its tactical nuclear attack capabilities and war deterrence 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 800 m (0.5 mile) under the scenario of a tactical nuclear attack, KCNA said.

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-U.S. combined military drills, calling it a rehearsal for invasion against it. The allies have been carrying out their annual exercises since earlier this month.

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.