WASHINGTON (AFP) - A powerful US military sea-based radar is now in place to detect any possible missile launches by North Korea, according to a senior US defense official.

"The SBX is in position," the defense official, who spoke on condition of anonymity, told AFP late on Wednesday.

The official would not offer more details but confirmed the SBX, a floating X-band radar that resembles a giant golf ball, had reached a location at sea where it could track missiles fired by the Pyongyang regime.

US and South Korean forces remain on heightened alert amid expectations North Korea will launch a missile or multiple missiles before the April 15 celebrations for the birthday of the North's late founder, Kim Il Sung.

The United States has already bolstered its missile defenses in the region to help protect allies South Korea and Japan as well as US bases in Guam.

Two US Arleigh-class destroyers have been sent to the western Pacific with anti-missile weaponry while ground-based THAAD interceptor batteries had been deployed to Guam, a US territory about 3,380km south-east of North Korea.

The US military also staged shows of force as part of drills with South Korea, with over-flights by B-52 and B-2 bombers as well as sophisticated F-22 fighter jets.

The floating X-band radar, which sits atop a platform similar to an oil rig, had been stationed at Pearl Harbour in Hawaii.

The SBX radar, which is 85m tall and operated by civilian contractors, is supposed to detect missile launches over a range of at least 2,000km.