TOKYO/ SEOUL: Japan put its military on alert on Monday for a possible North Korean ballistic missile firing, while South Korea also said it had detected evidence of launch preparations, officials from Japan and South Korea said.

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 ordered naval destroyers and anti-ballistic missile Patriot batteries to be ready to shoot down any projectile heading for Japan, Japan's NHK state broadcaster 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.

A Patriot missile battery on the grounds of Japan's Ministry of Defence had its missile tubes 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.

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

Japan has put its anti-ballistic missile forces on alert at least twice this year after detecting signs of launches by North Korea.

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

Last Friday, leaders of the Group of Seven industrialised nations, including Japanese Prime Minister Shinzo Abe and US President Barack Obama, met in Japan and demanded that North Korea comply with a UN Security Council resolution to stop all nuclear and missile tests and refrain from provocative action.

On the same day, North Korea threatened to retaliate against South Korea after it fired what it said were warning shots when boats from the North crossed the disputed sea border off the west coast of the Korean peninsula.

Japan has advanced Aegis vessels in the Sea of Japan that are able to track multiple targets and are armed with SM-3 missiles designed to destroy incoming warheads in space before they re-enter the atmosphere and fall to there targets.

Patriot PAC-3 missile batteries, designed to hit warheads near the ground, are deployed around Tokyo and other sites as a second and final line of defence.