It is the first US military response to Pyongyang’s claim that it could launch an intercontinental ballistic missile, CNN reported on Thursday.
The sea-based X-band radar (SB-X) is able to track the long-range launches and provide crucial data, the defence official said.
The radar has been deployed several times in the past to monitor North Korean missile activity.
But it can only remain at sea for a certain, undisclosed, period of time, so military officials try to calculate the most significant times, the official said.
Generally the SB-X is sent north of Hawaii and stationed about halfway to Alaska for the optimum spot to track a potential North Korean missile launch headed for Alaska, Guam or the West Coast of the US.
Additional surveillance assets are being identified to monitor activity on the Korean Peninsula, the official said but declined to provide any further details.
Earlier this week, North Korean leader Kim Jong-un said it could launch an intercontinental ballistic missile (ICBM) “at any time”.
In response, US authorities on Wednesday imposed sanctions on seven senior North Korean officials including Kim Jong-un’s younger sister Kim Yo Jong who is the Vice Director of the Workers’ Party of Korea Propaganda and Agitation Department (PAD).