SEOUL/WASHINGTON (REUTERS) - North Korea's latest missile test places Washington within range, but it still needs to prove critical missile technology, such as re-entry, terminal stage guidance and warhead activation, South Korea said on Friday (Dec 1).

Meanwhile in the US, experts say images released by North Korea on Thursday appeared to show it has succeeded in developing a missile, Hwasong-15, capable of delivering a nuclear weapon anywhere in the United States and it could be only two or three tests away from being declared combat ready.

Pyongyang has said its Wednesday missile test was a"breakthrough" and leader Kim Jong Un said the country had "finally realised the great historic cause of completing the state nuclear force".

"Kim Jong Un is acting in a very calculative, clever manner," said South Korea Defence Minister Song Young Moo. "Kim changed the launch time, direction and distance in order to display he has this great power.... he will probably make a great announcement in his New Year's Address that the North has completed its weapons programme."

South Korea's Ministry of Defence said the Hwasong-15 missile was a new type of intercontinental ballistic missile (ICBM) which can fly over 13,000km, placing Washington within target range. However, North Korea still needs to prove some technologies, like re-entry, terminal stage guidance and warhead activation, said Yeo Suk Joo, deputy minister of defence policy at the defence ministry.

According to an analysis by South Korea's military, Yeo told lawmakers at a parliamentary session the first stage engine of the Hwasong-15 missile was a clustering of two engines from Hwasong-14 missiles, which are also ICBMs that were test-launched in July this year.

Yeo said the Hwasong-15 is 2m longer than the Hwasong-14, while the second-stage engine requires further analysis. In order to curb further provocations from the North, US strategic assets will continue to be rotated on and near the Korean peninsula until the Pyeongchang Winter Olympics next February, Yeo said.

More tests unlikely anytime soon

South Korea's Unification Ministry said it believed that North Korea was unlikely to engage in more missile and rocket tests anytime soon due to a number of reasons, including the northern hemisphere winter season.

"For now if there are no sudden changes in situation or external factors, we feel there is a high chance North Korea will refrain from engaging in provocations for a while," said Lee Yoo Jin, deputy spokesman at the Unification Ministry.

North Korea is known to test fewer missiles in the fourth quarter of the year as troops are called to help with harvests, while the cold temperature is a strain on its fuel supplies. This week's missile launch was the first in 75 days.

The latest provocation from the North prompted more insults from US President Donald Trump, who referred to North Korea's leader Kim Jong Un as "Little Rocket Man" and a "sick puppy".

Trump had also dismissed a Chinese diplomatic effort to rein in North Korea's weapons programme as a failure on Thursday, while Secretary of State Rex Tillerson said Beijing was doing a lot, but could do more to limit oil supplies to Pyongyang.

Despite international condemnation and sanctions, North Korea has continued on its path towards developing a nuclear-tipped missile that could hit the United States. After North Korea released video footage and photographs of Hwasong-15, analysts have said they appeared to show the North was indeed capable of delivering a nuclear weapon anywhere in the United States and could only be two or three tests away from being combat ready.

South Korean President Moon Jae In said on Thursday in a phone call with Trump that the new missile was North Korea's most advanced so far, but it still has some technical issues to settle, like re-entry and terminal guidance system technology.

Almost combat ready

US-based experts, some of whom have been skeptical about past North Korean claims to have put all of the United States in range, said data from the latest test and the photos appeared to confirm North Korea has a missile of sufficient power to deliver a nuclear warhead anywhere in America.

The missile could be only two or three tests away from being declared combat ready, the experts said on Thursday.

Experts and US officials say questions remain about whether it has a re-entry vehicle capable of protecting a nuclear warhead as it speeds toward its target and about the accuracy of its guidance systems.

In an analysis for the Washington-based 38 North think tank, missile expert Michael Elleman of the International Institute for Strategic Studies said the North Korean photos showed a missile considerably larger than its predecessor.

"Initial calculations indicate the new missile could deliver a moderately sized nuclear weapon to any city on the US mainland," Elleman said. Elleman said the missile was large and powerful enough to carry simple decoys or other countermeasures to challenge US missile defences.

"A handful of additional flight tests are needed to validate the Hwasong-15's performance and reliability, and likely establish the efficacy of a protection system needed to ensure the warhead survives the rigors of atmospheric re-entry," Elleman wrote.

Only two or three more tests might be needed if North Korea could accept low confidence in the missile's reliability. Another missile expert, whose employer does not allow him to speak publicly to the media, agreed with Elleman's assessment.

"If North Korea does not make high demands on the reliability or accuracy of the missile... two or three more tests would suffice. "So long as North Korea can hit US cities with thermonuclear warheads, they probably don't need the ability hit every city they target or target specific aim points within those cities to convince the US leadership that war with North Korea would be too expensive to contemplate."

US based experts said North Korea had almost certainly developed a warhead light enough to be carried by the Hwasong- 15, which Elleman said should be capable of delivering a 1,000kg payload.

Larger, more powerful

Jeffrey Lewis of the Middlebury Institute of Strategic Studies said on Twitter the Hwasong-15 was "so big that the warhead wouldn't need to be miniaturised."

On Wednesday, US Defence Secretary James Mattis acknowledged the missile went higher than any previous test and was part of a North Korean effort "to continue building ballistic missiles that can threaten everywhere in the world, basically."

North Korea said the missile soared to an altitude of about 4,475km (2,780 miles), more than 10 times the height of the International Space Station, and flew 950km (590 miles) during its 53-minute flight before landing in the sea near Japan. The missile's large size was immediately apparent in the photos, which analysts said allowed for a more powerful propulsion system.

"This is a very big missile," said Michael Duitsman, a research associate at the Centre for Nonproliferation Studies."And I don't mean 'big for North Korea.' Only a few countries can produce missiles of this size, and North Korea just joined the club."

US experts and officials said the missile still appeared to be powered by liquid fuel, something that made it vulnerable as it could take to up to two hours to fuel on-site before launching. Earlier, one US official told Reuters the missile could have been powered by solid fuel, but experts said North Korea could still be a few years away from being able to field a more versatile solid-fueled ICBM.

The photos appeared to show the missile with at least two large nozzles on its first stage, instead of the one large and several smaller nozzles on the Hwasong-14.

"The first stage seems to use essentially the same case (as the Hwasong-14) but has two engines," said David Wright, of the Union of Concerned Scientists, a US-based nonprofit science advocacy group.

"The second stage looks like it can carry more than twice as much propellant. The combination of those two things means it really is a new, more capable missile."

While the photos show a mobile erector vehicle being used to position the missile upright, it is not seen in photos of the launch itself.

US intelligence analysts have concluded that the test missile was fired from a fixed position, not a mobile launcher, three US officials said.

The massive vehicle was "100 per cent" a domestic product of North Korea, state media quoted Kim Jong Un as saying. Western analysts said it is more likely the truck was one of about half a dozen vehicles obtained years ago from China, which North Korea has modified since then.