Japan has put its military on alert for a possible North Korean ballistic missile firing, while South Korea says it has detected evidence of launch preparations, officials from Japan and South Korea say.

Tension in the region has been high since North Korea conducted its fourth nuclear test in January and followed that with a satellite launch and test launches of various missiles.

Japan on Monday ordered naval destroyers and Patriot anti-ballistic missile batteries to be ready to shoot down any projectile heading for the country, state broadcaster NHK said.

A Japanese official, who declined to be identified as he is not authorised to speak to the media, confirmed the order. A spokesmen for Japan's defence ministry declined to comment.

The missile tubes on a Patriot missile battery on the grounds of Japan's Ministry of Defence were elevated to a firing position.

The South Korean defence official declined to comment on what type of missile might be launched, but South Korea's Yonhap News Agency said officials believe it would be an intermediate-range Musudan missile.

"We've detected a sign and are tracking that. We are fully prepared," said the South Korean official, who also declined to be identified.

A Pentagon spokesman, US Navy Commander Gary Ross, said: "We are closely monitoring the situation on the Korean Peninsula in coordination with our regional allies. We urge North Korea to refrain from provocative actions that aggravate tensions and instead focus on fulfilling its international obligations and commitments."

North Korea tried unsuccessfully to test launch the Musudan three times in April, according to US and South Korean officials.

North Korea's nuclear and missile tests this year triggered new UN sanctions. But it seems determined to press ahead with its weapons programs, despite the sanctions and the disapproval of its sole main ally, China.