North Korean leader Kim Jong Un has said an elevated U.S. security partnership with South Korea and Japan poses a grave threat to his country and vowed to further bolster his nuclear weapons program.

North Korean leader Kim Jong Un has called for bolstering nuclear forces this year during a visit to a nuclear material production base and nuclear weapons institute, state media KCNA reported on Wednesday.

North Korean leader Kim Jong Un has vowed to bolster his nuclear weapons program in response to the U.S.’s enhanced security partnership with South Korea and Japan. Last year, the U.S. expanded bilateral military exercises with South Korean and trilateral training with South Korea and Japan.