North Korea has executed its defence chief on treason charges after he fell asleep at an event attended by leader Kim Jong-un, South Korean media quoted Seoul's National Intelligence Service as saying in a briefing to lawmakers on Wednesday.

North Korean Defence Minister Hyon Yong-chol was purged and then executed by firing squad, media reported.

Hundreds of officials watched Hyon's execution on 30 April 30, Han Ki-beom, the deputy director of the National Intelligence Agency, told a parliamentary committee, the Yonhap news agency reported.

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Hyon, who was appointed to the post of Minister of the People's Armed Forces less than a year ago, was apparently caught dozing off during formal military events and also talked back to the dictator Kim on several occasions, Yonhap said.

The Yonhap report was based on a briefing by a ruling Saenuri party lawmaker who attended the parliamentary committee meeting.

Han told the committee that Hyon was executed with anti-aircraft fire – a method cited in various unconfirmed reports as being reserved for senior officials of whom the leadership wishes to make examples.

In North Korea, the defence minister is mainly in charge of logistics and international exchanges. Policy-making is handled by the powerful National Defence Commission and the party Central Military Commission.

The execution comes after South Korea's spy agency said late last month that Kim ordered the execution of 15 senior officials this year as punishment for challenging his authority.

No danger to Kim's brutal regime

"North Korean internal politics is very volatile these days," Michael Madden, an expert on the North Korean leadership and contributor to the 38 North think tank, told Reuters.

"Internally, there does not seem to be any respect for Kim Jong-un within the core and middle levels of the North Korean leadership," he added.

"There is no clear or present danger to Kim Jong-un's leadership or stability in North Korea, but if this continues to happen into next year, then we would seriously have to start looking at a contingency plan for the Korean peninsula."

In 2013, Kim purged and executed his uncle, Jang Song-thaek, once considered the second most powerful man in Pyongyang's leadership circle, for corruption and committing crimes damaging to the economy, along with a group of officials close to him.

Pyongyang's military leadership has been in a state of perpetual reshuffle since Kim took power.