North Korea defence chief has been executed by a firing squad using an anti-aircraft gun, according to South Korea’s National Intelligence Service.

Hyon Yong Chol death is the latest in a series of high-level purges since Kim Jong-un took control of the reclusive nation, 

Hyon, 66, who headed the isolated country's military, was charged with treason, including disobeying Mr Kim and falling asleep during an event at which North Korea's leader was present, according to South Korean lawmakers briefed in a closed-door meeting with the spy agency.

His execution was watched by hundreds of people, they said.

It was not clear how the NIS obtained the information and it is not possible to independently verify such reports from within secretive North Korea.

Experts on North Korea said there was no sign of instability in the country, but there could be if ongoing purges continued.

Mr Kim ordered the execution of 15 senior officials this year as punishment for challenging his authority, according to the NIS.

In all, around 70 officials have been executed since Mr Kim took over after his father's death in 2011, Yonhap news agency cited the NIS as saying.

"North Korean internal politics is very volatile these days. Internally, there does not seem to be any respect for Kim Jong-un within the core and middle levels of the North Korean leadership," said Michael Madden, an expert on the country's leadership and contributor to the 38 North think tank.

He added: "There is no clear or present danger to Kim Jong-un's leadership or regime stability, but if this continues to happen into next year, then we should seriously start to think about revising our scenarios on North Korea".

Koh Yu-hwan, a North Korea leadership expert at Dongguk University in Seoul, said the regime could "reach its limit" if Mr Kim's purges continued.

"But it's still too early to tell," Mr Koh said.

South Korean lawmakers said Hyon was executed at a firing range at the Kanggon Military Training Area, 22 km north of the capital city Pyongyang.

The US-based Committee for Human Rights in North Korea said last month that, according to satellite images, the range was likely used for an execution by ZPU-4 anti-aircraft guns in October.

The target was just 30 metres away from the weapons, which have a range of 8,000 metres, it said.

Hyon, last known to have spoken publicly at a security conference in Moscow in April, was said to have shown disrespect to Kim by dozing off at a military event, the Seoul lawmakers said, citing the agency briefing.

Hyon was also believed to have voiced complaints against Mr Kim and had not followed his orders several times, according to South Korea.

He was arrested late last month and executed three days later without legal proceedings, the NIS said.

In 2013, Mr Kim, the third generation of the his family to rule the country, purged and executed his uncle, Jang Song Thaek,

Jang was considered the second most powerful man in Pyongyang's leadership circle but was executed for factionalism and committing crimes damaging to the economy, along with a group of officials close to him.

North Korea's military leadership has been in a state of perpetual reshuffle since Kim Jong-un took power.

Mr Kim, who is in his early thirties, has changed his armed forces chief four times since coming to power.

His father Kim Jong-il, who ruled the isolated nuclear-capable country for almost two decades, replaced his chief just three times.