North Korea could conduct a third nuclear test or long-range missile launch this year as its new leader Kim Jong-Un tries to strengthen his authority, a South Korean think-tank said Wednesday.

There are also "ample possibilities" of limited attacks on the South by military commanders competing to demonstrate loyalty to Jong-Un, the Institute for Foreign Affairs and National Security (IFANS) said in a report.

The untested Jong-Un, aged in his late 20s, was proclaimed supreme leader after his father Kim Jong-Il died of a heart attack on December 17.

The institute, which is affiliated to the South's foreign ministry, said Jong-Un is likely to take "risky actions" to strengthen his regime by showcasing his own accomplishments.

It said he may stage missile and nuclear tests in the last half of 2012, when presidential elections are held in the United States and South Korea, or in the first half of 2013 when new administrations are settling in.

"In particular, Kim Jong-Un may attempt to stage an uranium bomb test in order to maximise international impact and showcase his own achievements," the report said.

The North in November 2010 disclosed an apparently operational uranium enrichment plant, which could give it a second way to make a nuclear bomb in addition to its plutonium-based weaponry.

Six-nation talks on the North's nuclear disarmament have been stalled since December 2008. The US and other negotiating partners say the North should shut down the uranium plant before negotiations resume.

IFANS said six-party talks could resume in the first half of this year but would make hardly any progress.

Inter-Korean ties have been icy since Seoul accused Pyongyang of torpedoing a warship which sank with the loss of 46 lives in March 2010. The North denied the charge but shelled a South Korean border island in November 2010, killing four.

The report said similar attacks were likely "any time".

The North under its new leader has kept up its harsh criticism of the South's conservative government.