SEOUL - New photographs of North Korean leader Kim Jong Un's visit to a key institute show that his country has made unexpected advances in its missile programme and possibly working on a more powerful solid-fuel ballistic missile, experts have said.

One photo of Kim reveals a poster on the wall clearly mentioning a missile called "Pukguksong-3", a potential successor to the previous two versions of the missile which were both solid-fuel, medium-range projectiles, reported CNN.

Unlike liquid-fuel rockets, solid-fuel missiles do not have to be loaded with fuel just before launching, a process that can take as long as an hour and make the missile vulnerable to a pre-emptive strike. Solid-fuel missiles are also easier to transport and hide.

All ballistic missiles owned by the United States and Russia are solid-fuel models, said Michael Duitsman, a research associate at the James Martin Center.

The new Pukguksong-3 missile was identified as an "underwater strategic ballistic missile," indicating that it would have a longer range than the Pukguksong-1, North Korea's only submarine-launched ballistic missile.

The Pukguksong-1 missile flew a record 500 km in a test on Aug 24 last year.

Submarine-launched ballistic missiles are relatively hard to detect, and a longer-range version of such a missile would give the North considerably more strategic leverage, military experts said.

South Korean officials have long feared that North Korea was building a more advanced Pukguksong missile.

Missile experts also highlighted another photograph of North Korean leader standing next to a large copper-coloured container, which experts said could be a wound-filament reinforced plastic rocket casing.

They said the wound-filament casing seen in the photo would be lighter than previous metal versions, allowing North Korea's missiles to fly further, according to the CNN report.

The ability to produce large wound-filament casings was crucial to the development of Soviet road-mobile ICBMs & IRBMs, Duitsman said in a tweet.

"This is the North Koreans showing us, or at least portraying, that their solid-fuel missile programme is improving at a steady rate," David Schmerler, research associate at the Monterey Institute of International Studies' James Martin Center for Non-proliferation Studies, told CNN.

Kim Dong Yub, a defence analyst at the Institute for Far Eastern Studies at Kyungnam University in Seoul, told New York Times: "It appears that the North is trying to tell the world that its re-entry and solid-fuel technologies are no longer experimental but have reached the stage of mass production.

"Though whether that's credible is another matter."

On Wednesday, North Korean state media KCNA announced that Kim had visited the country's Chemical Material Institute of the Academy of Defense Sciences at an undisclosed location. It was not clear when the visit took place.

"He instructed the institute to produce more solid-fuel rocket engines and rocket warhead tips by further expanding engine production process," the KCNA statement said.

Kim Jong Un also learned about the processes for manufacturing ICBM warhead tip and solid-fuel rocket engine and made a field survey of the process for manufacturing solid-fuel rocket engine and specified tasks and ways for normalising the production at a higher level, said the report.

China's Xinhua news agency reported that North Korea has a large coal mining industry which produces carbon and carbon compound material whose advanced products are believed to be capable of replacing heavy oil fuels used for launching rockets and missiles by other countries.

The institute was responsible for producing fiber and carbon compound materials used to build high-power rocket engines and so-called re-entry vehicles for ballistic missile warheads.

The latest pictures appeared to be aimed at dispelling doubts whether the North has mastered the re-entry technology. Building a reliable re-entry vehicle, which allows a warhead to survive the intense heat and friction of re-entering the atmosphere, is one of the most difficult hurdles to clear in building an intercontinental ballistic missile, or ICBM.

Kim Jong Un said his country had proved the re-entry mastery through its string of missile tests, including two ICBM launchings last month.

But missiles experts say video footage of its second and latest ICBM test on July 28 appears to show the Hwasong-14 missile breaking up before landing, indicating that Pyongyang may not yet have mastered the re-entry technology.

South Korean Vice Defence Minister Suh Choo Suk also said following the two ICBM tests that the United States and South Korea "do not believe North Korea has yet completely gained re-entry technology in material engineering terms".

During a nationally televised news conference last week, South Korea President Moon Jae In indicated that the North had not built a reliable ICBM.

"I think the red line is when the North completes an ICBM and weaponizes it by loading it with a nuclear warhead, and it is inching toward that critical red line," Moon said, warning that additional missiles tests by the North would invite tougher and "unbearable" sanctions from the United States.