Japan said Friday it would deploy Patriot missiles in Okinawa permanently from this month as part of its efforts to boost defence capability amid concerns over North Korea's missile threat.

Defence Minister Itsunori Onodera said his ministry would station Patriot Advanced Capability 3 (PAC-3) systems at two military bases in the southern Japanese island "at the earliest opportunity in April".

The ministry had earlier planned to deploy them by March 2015 but "we were considering advancing the plan that will protect people's lives and property against ballistic missiles," Onodera told a news conference.

The move came as the country remains on full alert ahead of an expected mid-range missile launch by North Korea.

South Korean intelligence says the North has prepared two mid-range missiles for imminent launch from its east coast, despite warnings from ally China to avoid provocative moves at a time of soaring military tensions.

The issue appeared to take on greater urgency on Friday after it was revealed a report compiled by the US military spy agency had suggested Pyongyang has the ability to launch a nuclear-armed ballistic missile.

Japan, where the armed forces have been authorised to shoot down any North Korean missile headed towards its territory, has already stationed PAC-3s in its capital to protect the 30 million people who live there.

In addition to PAC-3 batteries, Aegis destroyers equipped with sea-based interceptor missiles have been deployed in the Sea of Japan (East Sea).

Pyongyang's bellicose rhetoric has reached fever pitch in recent weeks, with near-daily threats of attacks on US military bases including in Japan and South Korea in response to ongoing South Korean-US military exercises.

The timing of the North's missile launch is still "unpredictable", Onodera said, "but we will continue to be on alert so that we can take action at any time".

Analysts said Tokyo's measures were purely precautionary and a mis-targeted missile that might end up falling uncontrollably towards Japanese territory was most likely what Tokyo was readying for.