North Korea showcases new nuclear warheads as US presence looms.

North Korea has revealed new, smaller nuclear warheads called Hwasan-31s, which experts say could be placed on intercontinental ballistic missiles capable of reaching the US. The announcement came as a US aircraft carrier strike group, led by the USS Nimitz, docked in South Korea following joint naval drills. North Korean leader Kim Jong Un also ordered his country to ramp up production of "weapon-grade nuclear materials." The state-run Rodong Sinmun newspaper called the drills an "open declaration of war" and said they bore signs of a "preemptive attack" against North Korea.