South Korea wants out of a deal it made with the US in 2001 that limits it to missiles with a top range of 186-miles and warheads no larger than 1,100 pounds.
Apparently fed up with threats from the North, the South Koreans want to build missiles capable of hitting targets anywhere in the North which means increasing the range limit to 1,000 kms. or 620-miles. South Korea already has an arsenal of cruise missiles able to strike anywhere in North Korea.
Washington has sought to limit South Korea's missile capability fearing expansion would trigger Pyongyang to expand its already considerable arsenal.