US President Barack Obama said there were signs that China was recalculating his policy towards North Korea out of frustration at leader Kim Jong-Un's nuclear brinkmanship.

In an interview with ABC News aired Wednesday, Obama said that the latest alarming threats from Pyongyang following its third nuclear test were also not necessarily more dangerous than others in its long history of belligerence.

The US leader said that it was "promising" that China, which had historically tolerated "misbehavior" by the North Koreans because it was worried about regime collapse, was changing its thinking.

"You're starting to see them recalculate and say, 'You know what? This is starting to get out of hand'," Obama said, although he did not give examples of a modified attitude from Beijing.

"And, so, we may slowly be in a position where we're able to force a recalculation on the part of North Koreans."

Obama also said that North Korea's latest behavior was not "necessarily" an alarming evolution in its behavior from its previous provocations towards the United States and its Asian allies.

"They've all been serious. Because when you're talking about a regime that is oppressive towards its people, is belligerent, has shown itself to sometimes miscalculate and do things that are very dangerous -- that's always a problem."

He said that under his administration, the United States had improved its missile defenses even as Pyongyang had advanced its missile technology, and pointed out that its nuclear arsenal pre-dated his administration.

The president also called on Pyongyang to take some confidence building measures to break the cycle of confrontation, which would prompt the United States and other players in the frozen six-party talks to respond.

"They could start by ending nuclear testing. They could start by ending some of this missile testing. There are a whole battery of confidence-building measures that they could engage in."

"One thing we've tried to do is to make sure that we're not going to reward bad behavior.

"There previously have been patterns where, you know, they bang the spoon on the table and then suddenly they get food aid, or they get other concessions.

"Then they come back to the table and negotiate a little bit, and then if they get bored they start -- provocative actions again. We've broken that pattern."

North Korea earlier confirmed it had shredded the 60-year-old armistice ending the Korean War, and warned that the next step was an act of "merciless" military retaliation against its enemies.

A lengthy statement by the North's armed forces ministry added to the tide of dire threats flowing from Pyongyang in recent days that have raised military tensions on the Korean peninsula to their highest level for years.

The North has also threatened to launch nuclear strikes against the United States and South Korea in response to fresh UN sanctions adopted after the North carried out its third nuclear test in February.