The official, speaking on condition of anonymity, said the radar, known as the Sea-based X-band radar (SBX), left on Monday and would reach its destination, about 2,000 miles (3,218 km) northwest of Hawaii, towards the end of January.
The radar is able to track ICBMs and differentiate between hostile missiles and those that are not a threat.
On Tuesday, U.S. Defense Secretary Ash Carter said the U.S. military might monitor a North Korean intercontinental ballistic missile test and gather intelligence rather than destroy it, as long as the launch did not pose a threat.
"If the missile is threatening, it will be intercepted. If it's not threatening, we won't necessarily do so," Carter said,
"Because it may be more to our advantage to, first of all, save our interceptor inventory, and, second, to gather intelligence from the flight, rather than do that (intercept the ICBM) when it's not threatening.Read more at Reuters