State media said it was the ninth and final test of the engine, indicating that a full test-fire of a new intercontinental ballistic missile could be conducted in coming months.

Kim oversaw the "important test" the official Korean Central News Agency (KCNA) reported Tuesday, with images showing the leader looking at the flame from the engine test with binoculars.

Another photo showed what appeared to be a red horizontal flame from the test.

It was a "ground jet test of high-thrust solid-fuel engine using the composite carbon fiber material," KCNA said, adding it was the "the last one in the development process".

The news agency quoted Kim as saying that the new rocket engine "heralds a significant change in expanding and strengthening the nuclear strategic forces" of North Korea.

The engine test came a week after the North unveiled its new Hwasong-20, billed as its next-generation ICBM.

The test "indicates the production of a solid-fuel engine to be used for the new ICBM," Yang Moo-jin, former president of the University of North Korean Studies in Seoul, told AFP.

By describing it as the "last" stage of development, Pyongyang was pointing to "a probable test launch of its new ICBM soon," Yang said, adding that a launch within this year was highly plausible.

North Korea has become one of Russia's main allies since it invaded Ukraine three-and-a-half years ago, sending thousands of troops and container loads of weapons to help the Kremlin push Ukrainian forces out of western Russia, following Kyiv's shock incursion last year.

Analysts have speculated that the North is receiving Russian technical support for its banned weapons and satellite programs in exchange, allowing for more rapid progress on its missile development projects.

With Russian assistance, North Korea's missiles could be reassessed from "'crude' to 'complete,'" Yang added.

- China trip -

The test also came days after Kim returned to North Korea from a trip to Beijing to attend a military parade marking Japan's surrender in World War II, where he stood side by side with his Chinese and Russian counterparts Xi Jinping and Vladimir Putin.

North Korea has for years staged test flights of long-range missiles apparently able to reach the continental United States.

Pyongyang has also rolled out solid-fuel variants that are easier to mobilise, conceal and launch rapidly compared with liquid-fuel missiles.

And with the nuclear-armed North seeking to use carbon-fiber material in its ICBMs, the weapons could gain greater range by becoming lighter, Hong Min, a senior analyst at the Korea Institute for National Unification, told AFP.

"By securing both light weight and thermal durability, it demonstrates the domestic development of essential materials for the extended range," he said.

North Korea has made repeatedly stated this year that it has no intention of giving up its nuclear weapons, and called South Korean President Lee Jae Myung a "hypocrite" over his remarks calling for a "path to denuclearisation".

"The North would remain unchanged in our stand not to abandon the nuclear weapons, the prestige and honor of the state," Pyongyang said in August.