North Korea is said to be restoring facilities at a long-range rocket launch site that it began to take apart last year during disarmament steps.
The reports in South Korean media follow the failed summit last week between US President Donald Trump and North Korean leader Kim Jong Un.
US National Security Adviser John Bolton has warned North Korea it must be willing to give up its nuclear weapons programme or it could face tougher sanctions.
Al Jazeera's Alexi O'Brien has more.
