The execution of the North Korean leader's uncle is "an ominous sign" raising concerns about instability in a nation pursuing a nuclear arms drive, US Secretary of State John Kerry warned Sunday.

Pyongyang on Thursday executed the uncle of North Korean leader Kim Jong-Un charging him with corruption and plotting to overthrow the state.

Kerry said the shock punishment of Jang Song-Thaek showed the world "how ruthless and reckless" Kim is, and he likened him to late Iraqi dictator Saddam Hussein.

"This is the nature of this ruthless, horrendous dictatorship and of his insecurities," Kerry told ABC television.

Jang's death -- just days after he was ousted from all his party and military positions -- marks the biggest political upheaval since the young Kim inherited power after the death of his father Kim Jong-Il in December 2011.

"It's an ominous sign of the instability and of the danger that does exist," Kerry said in the interview carried out during his trip to Vietnam.

The execution also underscored the importance of trying to rein in the reclusive regime's nuclear ambitions through the stalled six-party talks aimed at denuclearizing the Korean peninsula.

"To have a nuclear weapon, potentially, in the hands of somebody like Kim Jong (Un) -- just becomes even more unacceptable," Kerry said.

The United States was seeking to work with Pyongyang's close ally and neighbor China to find a way forward in the talks which have been on ice since 2008, he said.

North Korea has tested three nuclear bombs, most recently in February. Kim's regime has vowed to boost its nuclear "deterrent" but has said it would welcome a resumption of talks that previously promised aid for disarmament.

It is believed that Pyongyang has a small stockpile of crude nuclear bombs, but has not yet developed the capability to deliver them in warheads yet.

The United States and South Korea have long demanded that Pyongyang show a commitment to ending its nuclear weapons program before the six-party talks involving the two Koreas, Russia, China, Japan and the United States can resume.

"China is critical to any successful outcome with respect to denuclearizing North Korea. And we are now doing a more cooperative approach to the peninsula," Kerry said.

Veteran Republican Senator John McCain also called on Beijing to use its influence over Pyongyang, saying it had the power to do so.

"I think this young man is dangerous," McCain told CNN.

"I think it's obvious that this young man is capable of some very aberrational behavior," he added, voicing concerns "given the toys that he has."

Jang's execution highlighted "the instability, internally, of the regime, with the numbers of executions," Kerry maintained.

"This is not the first execution. There have been a significant number of executions taking place over the last months which we're aware of," Kerry told ABC.

Even though the outside world knows very little about the internal politics of the secretive regime, Kerry said Washington had gleaned a few insights about the young leader.

Kim was "spontaneous, erratic, still worried about his place in the power structure and maneuvering to eliminate any potential kind of a adversary or competitor and does so, obviously, ruthlessly," Kerry said.

Thursday's events showed the urgent need to get "China, Russia, Japan, South Korea, all of us to stay on the same page and to put as much effort into the denuclearization as possible."

Last week the State Department said it was increasing its talks with regional allies in the wake of the execution to try to find a way forward.

North Korea is under a punishing sanctions regime imposed by global powers and the United Nations, and it is thought that some of the nation's leaders are increasingly feeling economic pain.