North Korea has unveiled what appear to be tactical nuclear warheads as leader Kim Jong Un called for scaling up the production of weapons-grade nuclear material.

State-run Korean Central Television said Tuesday that Kim was briefed the day before by the Nuclear Weapons Institute on new nuclear arms development and the country's "nuclear weapon combined management system."

The TV program broadcast photos of what Pyongyang claims to be nuclear warheads. They also showed a panel featuring the term "Hwasan-31" as well as drawings of warheads mounted atop short-range ballistic and cruise missiles. "Hwasan" means volcano in Korean.

The broadcast said Kim examined plans and written orders for nuclear counterattack operations. He also reportedly issued a directive to expand the production of weapons-grade nuclear materials and spur the manufacture of more powerful nuclear weapons.

North Korea has reacted sharply against an ongoing joint landing exercise by the United States and South Korea, which follows another joint drill. The US has sent the nuclear-powered aircraft carrier USS Nimitz to the port of Busan.