North Korea unveils new nuclear warheads as US air carrier arrives in South

North Korea unveiled new, smaller nuclear warheads and vowed to produce more weapons-grade nuclear material to expand the country's arsenal, state media KCNA said on March 28, as a US aircraft carrier arrived in South Korea for military drills.

KCNA released photos of the warheads, dubbed Hwasan-31s, as leader Kim Jong Un visited the Nuclear Weapons Institute, where he inspected new tactical nuclear weapons and technology for mounting warheads on ballistic missiles, as well as nuclear counterattack operation plans, Reuters reports.

Experts say the images could indicate progress in miniaturising warheads that are powerful yet small enough to mount on intercontinental ballistic missiles capable of striking the US.

"It has something more powerful in a smaller space. That's worrisome," said Kune Y. Suh, professor emeritus of nuclear engineering at Seoul National University, comparing the new warheads to the 2016 version.

Kim Dong-yup, a former South Korean naval officer who teaches at Kyungnam University, said the warheads were most likely designed for use with at least eight different delivery platforms listed in posters on the wall, including missiles and submarines.

"Those are not limited to tactical missiles but appear to be a miniaturised, lightweight and standardised warhead that can mount on various vehicles," he said.

"Now that the delivery vehicles are nearly ready, they would churn out warheads to secure second-strike capabilities - perhaps hundreds, not dozens - while running centrifuges even harder to get weapons-grade nuclear material," he added.

Kim Jong Un ordered the production of weapons-grade materials in a "far-sighted way" to boost its nuclear arsenal "exponentially" and produce powerful weapons, KCNA said.

He said the enemy of the country's nuclear forces is not a specific state or group but "war and nuclear disaster themselves," and the policy of expanding the arsenal is solely aimed at defending the country, and regional peace and stability.

Kim was also briefed on an IT-based integrated nuclear weapon management system called Haekbangashoe, which means "nuclear trigger", whose accuracy, reliability and security were verified during recent drills simulating a nuclear counterattack, KCNA said.

North Korea has been ramping up tensions, firing short-range ballistic missiles on Monday and conducting a nuclear counterattack simulation last week against the U.S. and South Korea, which it accused of rehearsing an invasion with their military exercises.

North Korea's military simulated a nuclear airburst with two tactical ballistic missiles equipped with mock warheads during Monday's training while testing a nuclear-capable underwater attack drone again on March 25-27, KCNA said in separate dispatches.

The underwater drone, called Haeil-1, reached a target in the waters off the northeast coast after cruising along a "jagged and oval" 600km (373-mile) course for more than 41 hours, it said.

South Korean President Yoon Suk Yeol said North Korea does not deserve "a single penny" of economic aid while pushing for nuclear development, his spokesman said.

A South Korean military spokesman said that additional tests and analysis would be needed to verify whether the North's new warheads are deployable, but that its report on the underwater drone was most likely "exaggerated and fabricated".