Japan has wheeled missile defence systems into the heart of Tokyo after North Korea threatened to send a volley of rockets over the country towards Guam.

Pictures show how a PAC-3 Patriot missile unit has been moved in to a compound at the Defence Ministry in the capital after officials said they could shoot down North Korean rockets if they pass overhead. 

Sales of bomb shelters in Japan are said to have increased as tensions continue to rise in the region while officials in Seoul have vowed to bolster their defences. But South Korean citizens - long accustomed to its neighbour's fearsome rhetoric - are staying remarkably calm as the crisis unfolds, it has been reported.

Officials in Hawaii, meanwhile, say they are working on how to warn its 1.4million residents in the event of an attack. 

It comes after North Korea warned it will complete a plan to unleash an 'enveloping fire' around Guam by mid-August. This will involved firing four Hwasong-12 missiles over Japan and into waters near the tiny US island, officials claimed. Tokyo, which has in the past warned it would shoot down any North Korean missiles that threaten its territory, responded that it could 'never tolerate' provocations from the reclusive state. In April it emerged that sales of nuclear shelters and radiation-blocking air purifiers have surged in Japan  as North Korea pressed ahead with missile tests in defiance of U.N. sanctions.

A small company that specializes in building nuclear shelters, generally under people's houses, said it had received eight orders in April alone compared with six orders during a typical year.

The company, Oribe Seiki Seisakusho, based in Kobe, western Japan, also has sold out of 50 Swiss-made air purifiers, which are said to keep out radiation and poisonous gas, and is trying to get more, said Nobuko Oribe, the company's director.

Meanwhile, officials in Hawaii have revealed how they plan to respond in the event of a North Korean attack,CNN reports. 

Lt. Col. Charles Anthony, director of public affairs for the state's Department of Defense, said: 'If North Korea uses an intercontinental ballistic missile, from launch to impact (in Hawaii) is approximately 20 minutes;

Vern Miyagi, administrator for Hawaii's Emergency Management Agency, added: 'Pacific Command would take about fives minutes to characterize a launch, where the missile is going, which means the population would have about 15 minutes to take shelter.