By Ju-min Park and James Pearson

SEOUL (Reuters) - North Korea executed its defence chief by putting him in front of an anti-aircraft gun at a firing range, Seoul's National Intelligence Service told lawmakers, which would be the latest in a series of high-level purges since Kim Jong Un took charge.

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

His execution was watched by hundreds of people, according to NIS intelligence shared with lawmakers.

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.

"The NIS official said it had been confirmed by multiple sources. It is still just intelligence, but he said they were confident," Shin Kyoung-min, a lawmaker and member of the opposition New Politics Alliance for Democracy, who attended the briefing, told Reuters.

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

A spokesman for the U.S. State Department, Jeff Rathke, said he could not confirm the reports about Hyon but called them "disturbing."

"If they are true, (they) describe another extremely brutal act by the North Korean regime. These reports, sadly, are not the first in this regard," he told a regular news briefing.

Kim had previously ordered the execution of 15 senior officials this year as punishment for challenging his authority, according to the NIS. In all, some 70 officials have been executed since Kim took over after his father's death in 2011, Yonhap news agency cited the NIS as saying.

"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," said Michael Madden, an expert on the country's leadership who contributes to the 38 North think tank in Washington.

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

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

The lawmakers said Hyon, 66, was executed at a firing range at the Kanggon Military Training Area, 22 km (14 miles) north of Pyongyang, according to the NIS.

The U.S.-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 (100 feet) away from the weapons, which have a range of 8,000 metres, it said.

INSULAR, OPAQUE

Hyon was said to have shown disrespect to Kim by dozing off at a military event, the lawmakers said, citing the agency briefing. He was also believed to have voiced complaints against Kim and had not followed his orders several times, they said.

Hyon was arrested in late April and executed three days later without legal proceedings, the NIS told lawmakers.

"When the NIS is talking officially, they are relatively reliable. There are good reasons to believe it is true, but we cannot be 100 percent sure yet," said Andrei Lankov, a North Korea expert at Kookmin University in Seoul.

Last month, Hyon travelled to Moscow, where he spoke at a security conference. He was reported by North Korean state media to have appeared at an event in late April, an outward indication that all was normal.

North Korea is one of the world's most insular - and unpredictable - states and its power structure is highly opaque. The current leader is the third generation of the Kim family that has ruled with near-absolute power since the country was formed in 1948.

Kim Jong Un's father, Kim Jong Il, was appointed successor to the North Korean throne two decades before coming to power in 1994, during which time he was able to purge and alienate potential political challengers. Kim Jong Un, by contrast, has had few years to consolidate power.

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

Lankov said that the purges in Pyongyang did not necessarily point to instability.

"The common assumption is that it's bad for stability, but I'm not so sure," he said, adding it could be Kim consolidating his power base and removing people who had not sufficiently proved their loyalty.

Pyongyang's military leadership has been in a state of perpetual reshuffle since Kim Jong Un took power. He has changed his armed forces chief four times since coming to power, while his father, who ruled the country for almost two decades, replaced his chief just three times.

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 or punished, the lawmakers said.

Ma had also once served as vice director of the secretive Finance and Accounting Department in the ruling Workers' Party and, until recently, was effectively the regime's money man.

(Additional reporting by David Brunnstrom in Washington; Editing by Tony Munroe, Raju Gopalakrishnan and Christian Plumb)