North Korea's Kim orders production of more 'weapon-grade nuclear material'

North Korea's leader Kim Jong-un has called for his country to expand production of "weapon-grade nuclear materials" and build more powerful weapons, state media reported.

Kim's latest threat, a doubling-down on an earlier promise to ramp up nuke production "exponentially", came ahead of a US Navy carrier strike group's scheduled arrival in South Korea on Tuesday.

Kim was briefed by officials from the country's nuclear weapons institute, the official Korean Central News Agency reported, and said that North Korea should prepare to use its nukes "anytime and anywhere".

He called on officials to "expand on a far-sighted way the production of weapon-grade nuclear materials for thoroughly implementing the plan on increasing nuclear arsenals exponentially."

Kim also "put spurs to continuing to produce powerful nuclear weapons", KCNA added.

When North Korea has "flawlessly" prepared its nuclear weapons systems then "the enemy would fear us and not dare provoke our state sovereignty, system and people", Kim said, according to the report.

READ MORE: North Korea tests new underwater 'nuclear attack drone'

'Irreversible' nuclear power

North Korea last year declared itself an "irreversible" nuclear power and Kim recently called for an " exponential" increase in weapons production, including tactical nuclear weapons.

The country is also seeking to diversify its delivery mechanisms for its nuclear weapons, and on Tuesday claimed to have conducted a second successful test of its purported underwater nuclear attack drone.

The first test of the new weapon called Haeil – which means tsunami in Korean – was carried out last week in what Pyongyang said was a response to recent US-South Korea military exercises, the largest in scale in five years.

The new weapon manoeuvred underwater "for 41 hours and 27 minutes tracking a simulated route spanning 600 kilometres" before exploding a target off North Hamgyong province early Monday, KCNA said in a separate report.

The test "proved all strategic qualifications as well as safety and reliability in the weapons system", the KCNA report said.

Russia has also reportedly developed a similar weapon – the nuclear-capable Poseidon torpedo – but mastering the complex technology required for such weaponry might yet be beyond North Korea, experts have said.

The North Korean military also conducted a firing drill on Monday, simulating a nuclear attack using tactical ballistic missiles, KCNA said in a separate report.

READ MORE: North Korea fires two short-range ballistic missiles as tensions rise