Washington and Tokyo have agreed to deploy another missile defence radar in Japan, to counter the threat from North Korea, US Defense Secretary Leon Panetta said Monday.

"The purpose of this is to enhance our ability to defend Japan. It's also designed to help forward-deployed US forces," Panetta said of the X-band radar during a joint press conference in Tokyo with his opposite number.

"It also will be effective in protecting the US homeland from the North Korean ballistic missile threat," he added.

US officials insisted the powerful early warning system was not directed at China, but solely designed to counter North Korea.

About 47,000 US troops are stationed in Japan, many of them on the far southern island chain of Okinawa.

Japan already has one X-band radar, at Shariki base in Tsugaru City, in the far north of the main island of Honshu.

A US team is now on the ground looking for a site to locate the new radar, likely in southern Japan, a US defence official said earlier.

North Korea carried out a failed rocket launch in April in what it said was an attempt to put a satellite into orbit.

But the UN Security Council strongly condemned the exercise as breaching a ban on the testing of ballistic missile technology, and tightened sanctions.

The additional radar frees up US naval Aegis ships, which have been operating near Japan to help detect any potential incoming missiles, US officials said.