Japan has deployed Patriot missiles in its capital as it readies to defend the 30 million people who live in greater Tokyo from any North Korean attack, officials said Tuesday.

Two Patriot Advanced Capability-3 surface-to-air missile launchers were stationed at the defence ministry in Tokyo before dawn, a ministry spokesman said, while Defence Minister Itsunori Onodera said "we are proceeding with measures including deployment of PAC-3 as we are on alert".

Local reports said batteries would be deployed in another two locations in the greater Tokyo area.

"The government is making utmost efforts to protect our people's lives and ensure their safety," Prime Minister Shinzo Abe told reporters Tuesday morning.

"As North Korea keeps making provocative comments, Japan, cooperating with relevant countries, will do what we have to do.

"For the moment, the most important thing is to implement sanctions under the UN Security Council resolutions," Abe said.

Tokyo's response thus far to the threats emanating from Pyongyang has been low key and Tuesday's moves are the most visible yet that it is rattled.

PAC-3 batteries will also be installed in the semi-tropical island chain of Okinawa, Onodera told a television programme broadcast Monday.

He said Okinawa was "the place that is most effective in responding to emergencies... so we should deploy the unit in Okinawa on a permanent basis".

Japan's armed forces are authorised to shoot down any North Korean missile headed towards its territory, a defence ministry spokesman said Monday.

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

Tokyo's moves came as North Korea said Monday it was withdrawing all workers and suspending operations at a lucrative joint industrial zone with South Korea, with reports of heightened activity at the North's nuclear test site and at a missile battery.

North Korea'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 Korea-US military exercises.

Intelligence reports suggest Pyongyang has readied two mid-range missiles on mobile launchers on its east coast and plans a test-firing before the April 15 birthday of late founding leader Kim Il-Sung.

But Toshimitsu Shigemura, professor of international relations at Waseda University, said Tokyo's measures were purely precautionary and it was unlikely that Pyongyang would actually target Japan

"This is a verbal war and it's not accompanied by actual military actions," he told AFP.

"Government officials know from satellite images that Pyongyang has not mobilised its troops or weapons on the frontline, except that they moved mobile missile launchers to the east coast."

He said a mis-targeted missile that might end up falling uncontrollably towards Japanese territory was most likely what Tokyo was readying for.