North Korea boasted that the new intercontinental ballistic missile it just test-launched is "the world's strongest," a claim seen as pure propaganda after experts assessed it as being too big to be useful in a war situation.

The ICBM launched Thursday flew higher and for a longer duration than any other weapon North Korea has tested. But foreign experts say the test failed to show North Korea has mastered some of the last remaining technological hurdles to possess functioning ICBMs that can strike the mainland US.

The North's Korean Central News Agency identified the missile as a "Hwasong-19" and called it "the world's strongest strategic missile" and "the perfected weapon system" on Friday. The official media outlet said leader Kim Jong Un observed the launch, describing it as an expression of North Korea's resolve to respond to external threats to North Korea's security.

The colour and shape of the exhaust flames seen in North Korean media photos of the launch suggest the missile uses preloaded solid fuel, which makes weapons more agile and harder to detect than liquid propellants that in general must be fuelled beforehand.

But experts say the photos show the ICBM and its launch vehicle are both oversized, raising a serious question about their wartime mobility and survivability.

"When missiles get bigger, what happens? The vehicles get larger, too. As the transporter-erector launchers get bigger, their mobility decreases," Lee Sangmin, an expert at South Korea's Korea Institute for Defence Analyses.

The Hwasong-19 was estimated to be at least 28m long while advanced US and Russian ICBMs are less than 20m, said Chang Young-keun, a missile expert at Seoul's Korea Research Institute for National Strategy.

He suggested that the missile's size likely helped South Korean intelligence authorities detect the launch plan in advance.

"In the event of a conflict, such an exposure makes the weapon a target of a preemptive attack by opponents so there would be a big issue of survivability," Chang said.

Lee Illwoo, an expert with the Korea Defence Network in South Korea, said North Korea may have developed a larger missile to carry bigger and more destructive warheads or multi-warheads.

If that's the case, Lee said North Korea could have used liquid fuels as they generate higher thrust than solid fuels. He said some advanced liquid propellants can be stored in missiles for a few weeks before liftoffs.

Lee said North Korea may have placed a dummy, empty warhead on the Hwasong-19 to make it fly higher.

In recent years, North Korea has reported steady advancement in its efforts to obtain nuclear-tipped missiles. Many foreign experts believe North Korea likely has missiles that can deliver nuclear strikes on all of South Korea, but it has yet to possess nuclear missiles that can strike the mainland US.

The hurdles it has yet to overcome, according to experts, include ensuring its warheads survive the heat and stress of atmospheric re-entry, improving the altitude control and guidance systems for the missiles, and being able to use multiple warheads on a single missile to defeat missile defences.

"Acquiring reentry technology is currently the most important goal in North Korea's missile development, specifically for ICBMs, but they just keep increasing the ranges instead. This possibly suggests they still lack confidence in their reentry technology," Lee Sangmin said.

Chang said Friday's state media dispatch on the launch lacks details on the technological aspects of the Hawsong-19 and focused on publicity.

Other North Korean claims about its weapons capabilities have been met with wide outside scepticism.

In June, North Korea claimed to have tested a multiwarhead missile in the first known launch of such a weapon, but South Korea said the weapon instead blew up.

In July, when North Korea said it had test-fired a new tactical ballistic missile capable of carrying "a super-large warhead", South Korea said the claim was an attempt to conceal a botched launch.

North Korea's missile programme is still a major regional security concern, with the country openly threatening to use its nuclear missiles against its rivals. In a joint statement Thursday, the foreign ministers of South Korea, the US and Japan condemned the ICBM launch as a violation of UN Security Council resolutions and said they're committed to strengthening their efforts to block North Korea's illicit revenue generation funding its missile and nuclear programmes.

South Korea's Foreign Ministry said Friday it has imposed unilateral sanctions on 11 North Korean individuals and four organisations for their alleged roles in procuring missile components and generating foreign currency to fund Pyongyang's weapons program.

The sanctions are largely symbolic given that financial transactions between the Koreas have been suspended for years.

Observers say that Thursday's launch, the North's first ICBM test in almost a year, was largely meant to grab American attention days before the US presidential election and respond to international condemnation over North Korea's reported dispatch of troops to Russia to support its war against Ukraine.

North Korea's reported troop dispatch highlights the expanding military cooperation between North Korea and Russia.

South Korea, the US and others worry North Korea might seek high-tech, sensitive Russian technology to perfect its nuclear and missile programmes in return for joining the Russian-Ukraine war.