The National Intelligence Service (NIS) said on Wednesday the chief of North Korea's People's Armed Forces was executed for treason as well as disloyalty and disrespect toward North Korean leader Kim Jong-un.

At a closed-door briefing at the National Assembly’s Intelligence Committee, the NIS said Hyon Yong-chol was executed on April 30, just three days after his arrest and without facing a trial.

Hyon is said to have been executed with an anti-aircraft gun at Gang Gun Military Academy in Pyongyang with hundreds in attendance.

The NIS said Hyon was seen dozing off during an event attended by Kim on April 24 and was said to have often talked back to the young leader and disobeyed his commands.

Hyon is the highest ranking official to be purged since Kim’s uncle Jang Song-thaek was executed in December 2013.

Hyon held the number two position in North Korea’s military, after Hwang Pyong-so, chief of the General Political Bureau of the Korean People's Army.

Along with Hyon, three officials are known to have been purged: Han Kwang-sang, finance and accounting director at the ruling Workers’ Party of Korea; Ma Won-chun, head of the National Defense Commission’s planning department; and Byon In-son, director of operations at the military’s General Staff.

The three had often accompanied Kim to on-site inspections.

The NIS estimated that some 70 North Korean officials have been eliminated since Kim Jong-un rose to power in 2011.

The U.S. has called North Korea's execution of its defense chief brutal and disturbing, while European media said it shows the instability of the North Korean regime.

The U.S. State Department's Acting Deputy Spokesperson Jeff Rathke said at a daily press briefing Wednesday that he could not confirm the reports about Hyon Yong-chol, but if they are true, they describe another extremely brutal act by the North Korean regime.

Britain's BBC reported Wednesday that analysts say the execution of a figure as close to North Korean leader Kim Jong-un as Hyon was surprising and could give cause for concern about the country's stability.