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SEOUL, SOUTH KOREA -- North Korea's latest missile launches are a demonstration of the country's avowed ability to use nuclear force against South Korea and the mainland U.S. How immediate is that threat?

North Korea claims its nuclear forces are capable of destroying its rivals, and often follows its provocative weapons tests with launch details. But many foreign experts call the North's claim propaganda and suggest that the country is not yet capable of hitting the United States or its allies with a nuclear weapon.

There's no question that North Korea has nuclear bombs, and that it has missiles that place the U.S. mainland, South Korea and Japan within striking distance. What's not yet clear is whether the country has mastered the tricky engineering required to join the bombs and the missiles.

ICBMs

North Korea has demonstrated that it has missiles that could fly far enough to reach deep into the continental U.S., but it's not clear whether they can survive re-entering the earth's atmosphere on arrival.

North Korea said it had launched a Hwasong-15 intercontinental ballistic missile on Saturday to verify the weapon's reliability and combat readiness of the country's nuclear forces. It's one of three kinds of ICBMs the country has developed, along with the Hwasong-14 and Hwasong-17. All three are liquid-fueled, and North Korea has portrayed them all as nuclear-capable.

Launched almost straight up to avoid the territories of neighbours, the weapon reached a maximum altitude of about 5,770 kilometres (3,585 miles) and flew 990 kilometres (615 miles), according to North Korea's state media. The reported flight details suggest the missile could travel 13,000 kilometres (8,080 miles) or beyond if launched on a normal trajectory.

"These days, North Korea has been disclosing information about its launches in a very detailed manner to try to let others believe what they've done is genuine," analyst Shin Jong-woo at South Korea's Defence and Security Forum said. "But I think that's part of their propaganda."

There are questions on whether North Korea has acquired the technology to shield warheads from the high-temperature, high-stress environment of atmospheric re-entry.

A South Korean biennial defence document released last week said that it's not clear whether the missiles can survive re-entry, because all of North Korea's ICBM tests have so far been made on high angles.

Lee Choon Geun, an honorary research fellow at South Korea's Science and Technology Policy Institute, said a normal trajectory would cause greater stress, as a warhead would spend a longer time passing through altitudes of high air density.

North Korea's state media said the launch was made "suddenly" at a surprise order from leader Kim Jong Un.

"The Kim regime's claims of short-notice launches are thus intended to demonstrate not only the development of strategic and tactical nuclear forces but also the operational capability to use them," Leif-Eric Easley, a professor at Ewha University in Seoul, said.

In a military parade earlier this month, North Korea showcased around a dozen ICBMs, an unprecedented number that suggested progress in its efforts to mass-produce powerful weapons.

Among them were huge canister-sealed missiles that experts say were likely a version of a solid-fuel ICBM that North Korea has been trying to develop in recent years. Solid-fueled systems allow missiles to be mobile on the ground and make them faster to launch.

WARHEADS

North Korea likely has dozens of nuclear warheads. The question is whether they are small enough to fit on a missile.

North Korea has so far performed six underground nuclear test explosions to manufacture warheads that it can place on missiles. Outside estimates on the number of North Korean nuclear warheads vary widely, ranging from 20-60 to up to about 115.

In a 2021 interview with 38 North, a North Korea-focused website, renowned nuclear physicist Siegfried Hecker, who has visited North Korea's main Yongbyon nuclear complex numerous times, said that "20 to 60 is possible, with the most likely number being 45."

Some experts argue that North Korea has likely already built miniaturized nuclear warheads to be mounted on missiles citing the number of years the country has spent on its nuclear and missile programs. But others say North Korea is still years away from producing such warheads.

"After its sixth nuclear test, people accepted that North Korea really will have nuclear weapons. But they are still debating whether it has warhead miniaturization technology," Shin, the analyst, said.

The North described its sixth nuclear test in 2017 as a detonation of a thermonuclear bomb built for ICBMs. It created a tremor that measured 6.3 on the Richter scale, and some studies put its estimated explosive yield at about 50 to 140 kilotons of TNT. In comparison, the pair of atomic bombs dropped on Hiroshima and Nagasaki during World War II -- which killed a total of more than 210,000 people -- yielded explosions equivalent to about 15 and 20 kilotons of TNT, respectively.

The biennial South Korean defence document said that North Korea is estimated to have 70 kilograms (154 pounds) of weapons-grade plutonium. Some observers say that's enough for about 9-18 bombs. The document estimated that North Korea has "a considerable amount of" highly enriched uranium as well.

North Korea's Yongbyon complex has facilities to produce both plutonium and highly enriched uranium, the two main ingredients to build nuclear weapons.

Plutonium plants are generally large and generate a lot of heat, making it easier to detect. But a uranium enrichment plant is more compact and can be easily hidden from satellite cameras. North Korea is believed to be running at least one additional covert uranium enrichment facility, in addition to one at its Yongbyon complex.

SHORT-RANGE WEAPONS

Following the collapse of diplomacy with then-U.S. President Donald Trump in 2019, Kim sped up the development of short-range solid-fuel, nuclear-capable missiles designed to strike key targets in South Korea, including U.S. military bases there.

The so-called "tactical" nuclear weapons include what North Korea calls "super-large" 600-millimetre multiple rocket launchers that it tested Monday. South Korea describes the weapon as a short-range missile system.

North Korea's state media said its new artillery system can carry nuclear warheads, saying that four rockets would be enough to wipe out an enemy airfield. The statement drew quick outside doubts about whether weapons are indeed nuclear-capable.

"The North Korean claim doesn't make sense to some extent ... Why do they need four tactical nuclear weapons to destroy just one airfield?" Shin, the analyst, said. "Also, which country would disclose such attack scenarios via state media?"

Other new North Korean short-range systems include missiles that were apparently modelled after the Russian Iskander mobile ballistic system or outwardly resemble the U.S. MGM-140 Army Tactical Missile System. Launched from land vehicles, these missiles are designed to be maneuverable and fly at low altitudes, theoretically giving them a better chance of defeating South Korean and U.S. missile defence systems.

Whether North Korea has an ability to arm short-range missiles with nuclear warheads has not been independently confirmed.

While North Korea may be able to place simple nuclear warheads on some of its older missiles, including Scuds or Rodong missiles, it would likely require further technology advancements and nuclear tests to build smaller and more advanced warheads that can be installed on its newer tactical systems, said Lee, the expert.

North Korea also has an intermediate-range, nuclear-capable Hwasong-12 missile capable of reaching Guam, a major U.S. military hub in the Pacific. It has been developing a family of mid-range, solid-fuel Pukguksong missiles, which are designed to be fired from submarines or land vehicles.