United States secretary of state John Kerry has called the execution of North Korean leader Kim Jong-Un's uncle "an ominous sign" that raised concerns about instability in a nation pursuing a nuclear arms drive.

Kim Jong-Un on Thursday executed his uncle Jang Song-Thaek, charging him with corruption and plotting to overthrow the state.

Mr Kerry said the shock move showed the world "how ruthless and reckless" Mr 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," Mr Kerry said in an interview during his trip to Vietnam.

"It's an ominous sign of the instability and of the danger that does exist."

China, South Korea, Japan and the US are closely monitoring the situation, which is the biggest political upheaval since Kim Jong-Un inherited power after the death of his father Kim Jong-Il in December 2011.

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

"China is critical to any successful outcome with respect to denuclearising North Korea.

"And we are now doing a more cooperative approach to the peninsula."

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

"This is not the first execution," he said.

"There have been a significant number of executions taking place over the last months which we're aware of."

Mr Kerry said Washington had gleaned a few insights about the young leader.

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

Mr Kerry said the execution 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 denuclearisation as possible."

AFP