North Korean defector claims Kim Jong-un has ordered biggest missile yet for nation's 70th anniversary

File image provided by the North Korean government on November 30, 2017, of the Hwasong-15 intercontinental ballistic missile. Photo / AP

Kim Jong-un has ordered North Korean scientists to construct a missile that will be the largest in the regime's arsenal and to have it ready to be launched on September 9, the 70th anniversary of the founding of the republic, according to a defector.

His account suggests the North Korean leader issued the order at a two-day meeting of senior military leaders and scientists in Pyongyang in mid-December.

North Korean leader Kim Jong Un. Photo / AP

The defector has not been named, but was described by Japan's Mainichi newspaper as someone who was involved in the regime's missile programme and has retained links to contacts in the North.

He told the newspaper the missile will be designated the Unha-4 and will be a larger version of the Unha-3, a three-stage vehicle which North Korea claims is a rocket designed to put satellites into orbit.

An Unha-3 was successfully launched from the Sohae Satellite Launching Station, in the far north-west of the country, in February 2016.

The rocket is believed to have placed a small satellite in orbit, but experts have been unable to detect any transmissions from the satellite.

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North Korea insists it has the sovereign right to operate rockets and satellites, although the United Nations Security Council has condemned Unha-class rocket launches on the grounds that the vehicles are essentially modified long-range missiles and that the North is using the tests to advance its inter-continental strike capabilities.

North Korean state media appears to be readying for a new launch, with the Rodong Sinmun newspaper on December 25 stating in an editorial that: "Our satellite launch is a legitimate exercise of the right that thoroughly fits the UN Charter that enshrines the basic rights of respect for sovereignty and equality, and the international laws that govern the peaceful use of space."

The new rocket will be larger than the Unha-3, which is about 30 metres long and is based on the Soviet Union's Scud ballistic missile technology.

The defector claims that development of the body of the Unha-4 is effectively complete but that scientists will require a further six months to have the new weapon ready to launch.

It has been suggested that the rocket will be used to place a satellite in orbit to guide and observe future missile launches by the North, although it may also be used to test the ability of a warhead to survive re-entry into the atmosphere.

While North Korea is known to have made significant advances in its intercontinental ballistic missiles, analysts believe that Kim's scientists have still not full mastered shielding a warhead from the intense heat that builds up during re-entry.

The North Korean government called the Hwasong-15, a "significantly more" powerful, nuclear-capable intercontinental ballistic missile. Photo / AP

The reports coincide with the release of a study by the Sejong Institute of South Korea that warns that the North still needs to carry out test launches on a full ICBM trajectory and with a live warhead to demonstrate that it does have an effective nuclear deterrent.

"The North's seventh nuclear test could take place not underground but over the Pacific," Cheong Seong-chang, a senior analyst, said in the report.