Japan is set to purchase offensive air-to-surface missiles to counter North Korea's rising military threat, going against its own constitution, which dictates use of force in self-defence only.

Defence secretary Itsunori Onodera said the ministry intends to request a special budget for the fiscal year starting April 2018 to purchase long-range cruise missiles deployed on fighter jets.

The ministry reportedly plans to buy 560mile-range air-to-ground missiles from US firms, as well as Joint Strike Missiles with a range of some 310miles from Norway.

They will be the longest-range munitions of a country that has renounced the right to wage war.

Fighting back: Japan is set to purchase offensive air-to-surface missiles and Joint Strike Missiles in the wake of North Korea's missile tests (pictured)

The move will likely draw controversy as Japan's pacifist constitution bans the use of force as a means of settling international disputes.

Japanese military policy has so far been restricted to self-defence and relies heavily on the US to attack enemy territory under the Japan-US security alliance.

Its missile force had been limited to anti-aircraft and anti-ship weapons with a range of less than 190 miles. The new rockets will have a range of nearly 600 miles.

But Onodera insisted his ministry will continue to uphold the defence-only policy (which consists of 150,000 personnel), telling reporters: 'We will introduce them as standoff missiles that allow us to deal with our opponents from outside the range of threats.'

US President Donald Trump had caused consternation during his White House campaign by suggesting allies such as Japan need to do more to defend themselves, although since taking office Trump and his diplomats have offered reassurances of support.

Helping hands: Japan relies heavily on the US to attack enemy territory under the Japan-US security alliance. Pictured are US Air Force F-16 Fighting Falcons and F-35A Lightning during this week's military drill in South Korea

North Korea has threatened to sink Japan into the sea and has fired missiles over the country

Earlier this week, Prime Minister Shinzo Abe told parliament that North Korea's missile tests were an 'imminent threat' to Japan and talking to the reclusive state was meaningless.

The upper house unanimously adopted a resolution protesting against the North's firing of an intercontinental ballistic missile that dropped into the sea inside Japan's exclusive economic zone last week.

Global anxiety about North Korea has steadily risen this year, and Washington last week called on other UN members to cut ties with Pyongyang in order to squeeze the secretive regime.

The call, however, has fallen short of persuading key North Korean backers China and Russia to take steps to isolate the regime.

North Korea tested a new type of intercontinental ballistic missile last week that climbed to an altitude of more than 2,400 miles before crashing into the sea inside Japanese territorial waters.