North Korea is working on an advanced version of an existing missile that could reach the U.S., CNN reports. Citing a U.S. official, the report said Pyongyang is improving its current KN-20 intercontinental ballistic missile, just a few months after it launched its first ICBM. Several U.S. officials familiar with the matter said the work is just one small part of Kim Jong Un’s efforts to rapidly improve the country’s missile program. Pyongyang’s nuclear fuel, missile launchers, and guidance and targeting systems are also undergoing improvements, according to the report. Those improvements include solid rocket fuel with more stability to help missiles achieve greater distances, improved rocket motors and engine components, and improved targeting and guidance systems, according to CNN.