Seoul: North Korea has executed its defence chief on treason charges, Seoul's National Intelligence Service was quoted as telling lawmakers, in the latest of a series of high-level purges since Kim Jong Un took power after his father's death in 2011.

Hyon Yong Chol, who heads the isolated country's military, was purged and then executed by firing squad with an antiaircraft gun, watched by hundreds of people, South Korean media reported on Wednesday, citing the NIS's comments to a parliamentary panel.

Hyon, who spoke at a security conference in Moscow in April, was said to have shown disrespect to Kim by dozing off at a military event, media said, citing the agency briefing.

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.

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

‘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.

Hyon, a little-known general, was promoted to the rank of vice marshal of the North Korean army in 2012.

The South Korean spy agency told lawmakers that Ma Won Chun, known as North Korea's chief architect of new infrastructure under Kim, was also purged, local media reported.