New images reveal the forces using dog-like robots as 370 troops from the U.S. and South Korea take part in a four-day joint military exercise in Paju, a small city near the North

North Korea said Friday it test-launched new anti-aircraft missiles, as its military threatened unspecified grave steps against the U.S. and South Korea over their joint military drills that it views as an invasion rehearsal.

North Korean leader Kim Jong Un oversaw the test-firing of the country's latest anti-aircraft missile system on Thursday, state media KCNA reported on Friday. Kim thanked what was referred to as a research group for the system.