The U.S. Defense Department official said the monitoring by the destroyer USS John S. McCain and the SBX-1 radar would include any new missile launches by North Korea, CNN reported.
North Korea has been stepping up its dangerous threats and provocations against the United States and South Korea in response to tightened U.N. Security Council sanctions against it and the ongoing U.S.-South Korean joint military exercises.
Separately, Kyodo News quoted a Pentagon official that the U.S. Navy has sent an Aegis warship to waters southwest of South Korea.
As the North Korean threats increase, the United States also has recently sent its nuclear-capable B-2 stealth bombers, B-52s and F-22 Raptor stealth fighters as part of the exercises in a show of support to its ally South Korea.
On Monday, South Korea's new President Park Geun-hye asked her military to respond strongly "without any political considerations" if there is any provocation against South Korea and its people. That warning came after North Korea over the weekend announced it had entered a "state of war" against South Korea.
Pentagon spokesman George Little asked that the ship deployment not be connected to the military exercises in South Korea, CNN reported.
"I would urge everyone to disconnect this ship deployment from recent military exercises in South Korea. We have regular ship movements in the Asia-Pacific region and we use our ship movements for any number of purposes," he told CNN.
Little said recent U.S. activities with South Korea "have been about alliance assurance, about ensuring them that we are there to protect them," CNN reported.