North Korean leader Kim Jong Un sent a letter to South Korean President Moon Jae-in saying the incident should not have happened, an official said.

North Korean leader Kim Jong Un has warned of impending tests of a nuclear warhead explosion and ballistic missiles capable of carrying atomic warheads, state media reported Tuesday, in an escalation of threats against Seoul and Washington.

South Korea's president warned Tuesday that North Korea faces collapse if it doesn't abandon its nuclear bomb program, an unusually strong broadside that will likely infuriate Pyongyang.