North Korea is developing an advanced version of an intercontinental ballistic missile that could reach the United States, according to a report.

A US official reportedly told CNN of Kim Jong Un’s plans for an upgraded ICBM as part of the regime’s accelerated effort to improve its nuclear weapons capability.

It comes less than six months after North Korea launched its first intercontinental ballistic missile, and two months after its sixth nuclear test.

North Korea’s series of nuclear and missile tests are in defiance of UN Security Council resolutions and the rogue state has made no secret of its plans to perfect a nuclear-tipped missile capable of reaching the US mainland.

It regularly threatens to destroy the United States and its “puppet”, South Korea.

South Korea’s spy agency have also reported on North Korea’s plans for a new missile test, after it spotted increasing activity at its nuclear testing site in Pyongyang just days before US President Donald Trump’s visit to Seoul.

“There is a possibility of a new missile launch given the active movement of vehicles around the missile research institute in Pyongyang. The North will constantly push for further nuclear tests going forward, and the miniaturisation and diversification of warheads,” Reuters reported South Korea’s spy agency saying.

Defense Secretary James Mattis has also been sounding the alarm, noting recently, “North Korea has accelerated the threat that it poses to its neighbours and the world through its illegal and unnecessary missile and nuclear weapons programs,” he added, emphasising that the US “does not accept a nuclear North Korea,” CNN reported.

US bombers have since flown over the Korean Peninsula and carried out bombing drills simulating attacks on major targets in the country, the North’s official KCNA news agency says.

The strategic bombers, escorted by US and South Korean fighter jets, flew from the Anderson Air Force Base in Guam, to conduct a “surprise” strike drill, the KCNA said.

Two Guam-based US B-1B bombers, accompanied by fighter jets from South Korea and Japan, carried out the exercise in the vicinity of the Korean peninsula on November 2, the US Air Force said.

“The bilateral continuous bomber presence (CBP) mission was planned in advance … and was not in response to any current event,” the Air Force said in a statement.

North Korea has not launched any missiles since firing one over Japan on September 15, the longest such lull this year.

It is believed the missile research institute could have been damaged after Pyongyang’s sixth atomic detonation on September 3, according to Japanese media.

The Punggye-ri test site is thought to have been severely destabilised. Photo: DigitalGlobe/38 North

Japanese broadcaster TV Asahi, citing unnamed sources, said on Tuesday a tunnel at the test site collapsed after that explosion, possibly killing more than 200 people.

Experts have said a series of tremors and landslides near the nuclear test base probably mean the country’s sixth and largest blast on September 3 has destabilised the region.

The Punggye-ri nuclear site may no longer be used to test nuclear weapons, they said.

– with wires