North Korea is possibly planning new missile and nuclear tests, following signs of "active movement" at a missile research facility, the South Korean intelligence agency reports.

South Korea's spy agency raised the possibility on Thursday of further nuclear and missile tests from the North after brisk activity was spotted at its research facilities in Pyongyang.

It comes as US President Trump prepares to undertake an 11-day trip to Asia including a visit to South Korea in which he will visit US and South Korean forces, and hold bilateral talks with President Moon Jae In.

President Donald Trump. Photo / AP
President Donald Trump. Photo / AP

Reclusive North Korea has carried out a series of nuclear and missile tests in defiance of UN Security Council resolutions but has not launched any missiles since firing one over Japan on September 15, the longest such lull this year.

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However, a flurry of activity including the movement of vehicles has been detected at the North's missile research facilities in Pyongyang, where the most recent missile test was conducted, pointing to another possible launch, South Korea's Intelligence Service said in a briefing to politicians. It did not say how the activity was detected.

"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," the intelligence agency said.

Trump will visit five Asian nations in coming days for talks in which North Korea will be a major focus.

The trip, from November 3 to 14, will be designed to reinforce US commitment to leadership in a "free and open" Indo-Pacific region, the White House said.

Trump will visit Japan over the weekend followed by South Korea next week. There, he will visit American and South Korean military and hold talks with the South's leader.

He will also visit China and Vietnam, on a trip that comes as US allies have watched with increasing worry as Trump and North Korea have exchanged bellicose rhetoric.

The Hwasong-12 lifts off from the launching pad at an undisclosed location near Pyongyang in August. Photo / AP
The Hwasong-12 lifts off from the launching pad at an undisclosed location near Pyongyang in August. Photo / AP

North Korea 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.

It comes amid reports North Korea is working on a devastating new intercontinental ballistic missile (ICBM) capable of reaching the US, following a previous launch North Korea declared a "success" at the time.

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The development would be another step in Kim Jong-un's nuclear escalation. The North Korean leader has fired more missiles this year than his father did in his entire lifetime.

North Korea's rapid escalation has helped the rogue nation show off its sophisticated nuclear arsenal to the world - which has also led to the US to increase its missile defence systems.

A Washington Post investigation revealed US intelligence officials estimate that Kim Jong-un's nuclear arsenal is up to 60 weapons strong.

While the number is still concerning, it pales in comparison with Russia's estimated 7000 nuclear warheads, and the US' 6800.

However, North Korea has managed to "miniaturise" nuclear warheads so they can be put on intercontinental ballistic missiles - which is where the real problem lies.

According to officials who spoke to CNN, the US is adamant North Korea could perfect "miniaturising" their warheads by 2018.

Considering 2018 is less than two months away, the US has stepped up funding and research into its own missile-defence systems.

Fort Greely - located more than 500km north of Anchorage, Alaska - is regarded as the last line of defence against a North Korean missile attack and its systems stand ready to protect the US.

North Korean leader Kim Jong Un, centre, at an undisclosed location in North Korea. Photo / AP
North Korean leader Kim Jong Un, centre, at an undisclosed location in North Korea. Photo / AP

Heavily guarded, its 38 silos contain ground-based missile interceptors (GBIs) which have the ability to shoot down a potential missile. It sits 6115km away from Pyongyang, the capital city of North Korea.

By the end of 2017, the US is "expected to finish both a so-called Nuclear Posture Review and a Ballistic Missile Defence Review that will set the stage for billions of dollars and decades of new spending on modernised US weaponry to counter the type of threat North Korea poses", officials claim.

Despite the back and forth between North Korea and the US, military and intelligence circles from America claim there is a firm understanding that a nuclear-tipped ICBM would not be allowed to proceed.

Intelligence officials believe they would be tipped off if North Korea was to follow through on its nuclear threat.

And with new reports claiming North Korea is producing its own unsymmetrical di-methyl-hydrazine (UDMH), the liquid rocket engine fuel needed to launch Kim Jong-un's new generation of missiles, the rogue nation might be closer to escalation than originally thought.

UDMH was the fuel in the propellant combination used to launch the Hwasong-12 intermediate-range ballistic missile (IRBM) and Hwasong-14 intercontinental ballistic missile (ICBM).

The development of the Hwasong-12 IRBM and Hwasong-14 ICBM missiles and tightening sanctions have seen demand for the vital product increase.