North Korea appears to have made "some progress" in its pursuit to develop a hypersonic missile, Seoul's defense chief said Tuesday, after Pyongyang claimed to have successfully tested a solid-fuel one earlier this week.

Defense Minister Shin Won-sik made the remark in a radio interview with broadcaster KBS after the North launched an intermediate-range missile into the East Sea on Sunday in its first ballistic missile test this year.

Shin said the latest launch involved a conical-shaped warhead like the hypersonic missile the North tested in January 2022.

"The difference with the 2022 (one) is that was a liquid-propellant missile, while this time it was a solid propellant one that they are newly developing," he said. "(We) assess that there has been some progress."

Solid-fuel missiles are known to be harder to detect ahead of launch compared with liquid-fuel ones that require more pre-launch preparations, such as the injection of fuel.

A hypersonic weapon is among the list of high-tech weapons that North Korean leader Kim Jong-un vowed to develop at a party congress in 2021.

Meanwhile, Shin renewed the South's warning that the North Korean leadership will face its end if it makes a "wrong decision" amid North Korea's recently hardening rhetoric against the South.

The North's leader called for revising the country's constitution to define South Korea as its "invariable principal enemy" and to codify the commitment to "completely occupying" South Korean territory in the event of war at a parliamentary meeting Monday.

Shin assessed the North's harsh rhetoric as being intended to bolster internal unity in the face of economic difficulties.

He pointed to the North's recent arms exports to Russia as a sign that it would not actually risk war with the South.

"Among the missiles being developed by North Korea, the new ones are the so-called Iskander-type one and the 600 millimeter large-caliber multiple launch rocket system," he said. "Tens of these -- almost all of them that were produced were sold immediately to Russia."

"A barking dog does not bite," he said. "If North Korea really intended to go to war, would it be able to export millions of artillery shells to Russia, and also export its best-performing missiles as soon as they were made?"

The minister has said the North has sent around 5,000 containers to Russia as of end-December, which can accommodate some 2.3 million rounds of 152 mm shells.

Shin also touched on recent reports of the South not conducting proper inspections of North Korea's demolition of guard posts inside the Demilitarized Zone separating the Koreas under a 2018 inter-Korean military agreement.

The minister said there is a high possibility that the underground facilities of the North's destroyed guard posts were untouched as the North has recently been seen quickly restoring them, noting that documents on the inspections remain.

In late 2018, the two Koreas destroyed 10 guard posts each and withdrew from another one each to follow through on the military accord signed under the previous liberal President Moon Jae-in to reduce tensions along the border. (Yonhap)