North Korea's Kim Jong-Un faces no threat to his rule despite the sacking of his military chief, but also shows no sign of seeking reforms or easing confrontation, a leading think-tank said Wednesday.

"He could well be around for decades -- and with a growing nuclear arsenal," the International Crisis Group (ICG) said in a report entitled "North Korean Succession and the Risks of Instability".

Pyongyang, it said, may even decide to test more long-range missiles or another nuclear bomb when key nations like China and the United States are preoccupied with leadership changes or elections in coming months.

Last week's announcement that military chief Ri Yong-ho had been relieved of all posts, purportedly because of illness, raised some speculation of a military power struggle or of efforts by Kim to set the stage for reform.

The Brussels-based ICG, however, said that despite the sudden sacking there were no signs of opposition to the country's second dynastic succession and Kim appeared in charge in his own right.

There was also nothing to suggest that Kim, who took power after the death of his father Kim Jong-Il last December, would take measures to improve the lot of his people or reduce regional frictions.

The North has suffered acute food shortfalls since the early 1990s and severe shortages of electricity and materials cripple industry.

The ICG said the poor economy remained the regime's greatest long-term threat. But reform would contradict the centrally-planned system espoused by Jong-Un's father and grandfather, undermining his own political legitimacy.

While the North was currently stable, "the system is not sustainable forever", the ICG said.

"Meanwhile, reinforcing the status quo will not bring prosperity, only more backwardness and oppression for millions of North Koreans."

Continued isolation and a "military first" orientation would encourage the regime to remain confrontational.

Without the money to sustain a conventional arms race, it would rely increasingly on "asymmetric capabilities" including nuclear weapons for its security.

The ICG also cautioned that eventual internal opposition to Jong-Un's succession could not be ruled out.

If the armed forces became dissatisfied with the status quo, there were fears the regime could stage provocations against South Korea, as in 2010, to bolster the young ruler's military credentials.

Regarding former military chief Ri, the ICG said speculation he may have been removed due to a corruption scandal was plausible.

It said it was unlikely Ri had been plotting against the Kim family since this "would have brought deadly retribution and a media announcement that he had 'died in an accident'".