Japanese citizens will have just 10 minutes to prepare a North Korean ballistic missile attack, authorities in the country have warned.

In the event of an attack, a document posted on the country's civil protection site advises people to find the strongest concrete building possible or go underground.

Then they should then take cover under tables and stay away from windows, it says.

A ballistic missile would likely take around 10 minutes to travel 1,600 km (1,000 miles) from its launch pad in North Korea to Okinawa, it adds, citing a launch in February last year which took that length of time to fly over the Japanese island.

The warning comes as tensions ratcheted up between North Korea, its Asian neighbours and the US. ​The secretive communist state test-fired four ballistic missiles last month, three of which landed in Japan’s exclusive economic zone in the Sea of Japan, just off the coast of the country.

It has subsequently emerged that the country's civil defence website had 5.7 million visitors in the first 23 days of April — more than 14 times the usual monthly traffic.

North Korea marks 85th anniversary of army by showing images of missiles in government

Japan's early-warning system, which issues missile strike alerts to the population via loudspeaker and telephone has come under increased scrutiny amid the rising tensions.

In 1998, North Korea demonstrated that its missiles were capable of reaching Japan when it fired a missile to launch a satellite across Japanese territory that landed in its economic zone on the Pacific Ocean side.

Japan's government has been briefing local authorities on what they should do if a missile lands in their area and urging them to hold evacuation drills.

In pictures: North Korea marks the Day of the Sun Show all 16 1 /16 In pictures: North Korea marks the Day of the Sun In pictures: North Korea marks the Day of the Sun North Korean women soldiers take part in a military parade in Pyongyang on 15 April AFP/Getty Images In pictures: North Korea marks the Day of the Sun A huge military parade in Pyongyang marks the 105th birth anniversary of Kim Il Sung, the country's late founder and grandfather of current ruler Kim Jong Un AP In pictures: North Korea marks the Day of the Sun North Korean leader Kim Jong-un waves during a military parade on 15 April 2017 AP In pictures: North Korea marks the Day of the Sun North Korean soldiers carry flags and a photo of Kim Il-sung during a military parade in Pyongyang on 15 April AP In pictures: North Korea marks the Day of the Sun Soldiers march across Kim Il Sung Square during a military parade on 15 April AP In pictures: North Korea marks the Day of the Sun What military experts say appears to be a North Korean KN-08 inter-continental ballistic missile is paraded across Kim Il Sung Square during a military parade on 15 April AP In pictures: North Korea marks the Day of the Sun Women wearing traditional Korean dress wave flowers and shout slogans as they pass Kim Jong-Un during a parade in Pyongyang on 15 April AFP/Getty Images In pictures: North Korea marks the Day of the Sun North Korean female soldiers march during a parade for the 'Day of the Sun' festival on Kim Il Sung Square in Pyongyang on 15 April EPA In pictures: North Korea marks the Day of the Sun Military vehicles carry missiles with characters reading 'Pukkuksong' during a military parade marking the 105th birth anniversary of Kim Il-sung in Pyongyang on 15 April 2017 EPA In pictures: North Korea marks the Day of the Sun Attendees carry sheets in the colours of North Korea's national flag during a military parade in Pyongyang on 15 April Reuters In pictures: North Korea marks the Day of the Sun A soldier salutes from atop an armoured vehicle during a military parade in Pyongyang on 15 April Reuters In pictures: North Korea marks the Day of the Sun A North Korean woman cries as she looks towards Kim Jong-un during a parade on 15 April AP In pictures: North Korea marks the Day of the Sun University students carry the national flag and two bronze statues of the late leaders Kim Il-sung and Kim Jong-il during a military parade on 15 April AP In pictures: North Korea marks the Day of the Sun Missiles are driven past Kim Jong-un and other high ranking officials during a military parade marking the Day of the Sun in Pyongyang on 15 April Reuters In pictures: North Korea marks the Day of the Sun North Korean leader Kim Jong-un waves from a balcony during a parade for the 'Day of the Sun' festival in Pyongyang on 15 April 2017 EPA In pictures: North Korea marks the Day of the Sun Missiles are driven past Kim Jong-un and other high ranking officials during a military parade marking the Day of the Sun in Pyongyang on 15 April Reuters

Sales of nuclear shelters and radiation-blocking air purifiers have also surged in recent weeks.

Meanwhile, in the US, every member of the Senate was called to a White House briefing on the threat posed by North Korea last week.

Donald Trump triggered speculation he was planning a response in the event of another nuclear test by Pyongyang when he made hastily scheduled calls to Japan’s Prime Minister, Shinzo Abe, and China’s President, Xi Jinping, over the weekend.