North Korea reportedly executed its defense chief by putting him in front of an anti-aircraft gun at a firing range, Seoul's National Intelligence Service (NIS) told lawmakers.

Hyon Yong-chol, who headed the isolated nuclear-capable country's military, was charged with treason, including disobeying North Korean leader Kim Jong-un 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, Reuters reported. It was not clear how the NIS obtained the information and it is not possible to independently verify such reports from within North Korea.

Lü Chao, a research fellow of Korean studies at the Liaoning Academy of Social Sciences, told the Global Times that Kim has been revamping the military leadership but it's not possible that Pyongyang would execute a high-level official with an anti-aircraft gun.

"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.

Hyon was said to have shown disrespect to Kim by dozing off at a military event and was also believed to have voiced complaints against Kim and had not followed his orders, the Seoul lawmakers said.

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 in 2011, Yonhap news agency cited the NIS as saying.

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

Lü said that the South Korean report is not credible, citing earlier rumors that Jang Song-thaek was bitten to death by dogs. "The report is to defame North Korea," he said. A poll on the Global Times website huanqiu.com showed that over 50 percent of 6,000 netizens don't believe the South's report.

Pyongyang has been unsatisfied with NIS intelligence. In March, after Pyongyang announced the arrest of two South Korean spies, an article was published on uriminzokkiri.com, saying that NIS is the cradle of terrorism and its despicable acts have gone beyond the imagination of 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 meters away from the weapons, which have a range of 8,000 meters, it said.