North Korea has executed its defense chief on charges of treason

North Korea has executed its defense chief on charges of treason, South Korea's spy agency said Wednesday, in the latest sign of a reign of terror by leader Kim Jong-un.

Hyon Yong-chol, the chief of North Korea's People's Armed Forces, was executed by firing squad using an anti-aircraft gun at a military school in Pyongyang around April 30, the National Intelligence Service (NIS) said.

Hyon, 66, was seen dozing off during a military event and did not carry out Kim's instructions, a senior official of the agency told a group of reporters.

The spy agency also gave a similar briefing to lawmakers in a closed-door parliamentary session on Wednesday.

Hyon's execution is the latest in a series of public executions in the communist country.

Hyon was named as the armed forces chief in June 2014, the No. 2 man within the North's military after Hwang Pyong-so, director of the general political department of the Korean People's Army (KPA). North Korea has not announced its purge of Hyon yet.

Hyon Yong-chol, the chief of North Korea's People's Armed Forces (R), has been executed as he was "disrespectful" to the North's young leader Kim Jong-un (L), South Korea's spy agency said on May 13, 2015. This file photo, taken on Feb. 16, 2015, shows that they visited the Kumsusan Palace of the Sun to commemorate the birthday of late leader Kim Jong-il. (Yonhap file photo)

The NIS said that given available information, Hyon seemed to be purged not because he sought a rebellion but because he was "disrespectful" to the young leader.

Over the past six months, Kim punished other key senior officials including Ma Won-chun, director of the Designing Department at the North's powerful National Defense Commission.

"As key officials have voiced more complaints, Kim has deepened a reign of terror by purging them in negligence of proper procedure," the official said. "We believe that there are growing doubts about Kim's leadership among North Korean ranking officials."

   The Ministry of Unification said that the move is aimed at consolidating the young leader's power through a reign of terror.

"North Korea is seeking to create an atmosphere of terror by employing such ways of execution in order to solidify his power," ministry spokesman Lim Byeong-cheol said at a regular briefing. "The government is closely watching how the young leader's governing style will affect the regime in the long run."

   The spy agency said last month that North Korea has killed another 15 senior officials this year.

North Korea has often carried out public executions in what critics say is aimed at instituting a reign of terror to consolidate Kim's grip on power that he inherited upon the death of his father and long-time leader, Kim Jong-il, in 2011.

In 2013, Kim ordered the execution of his once-powerful uncle, Jang Song-thaek, who was accused of treason, a shocking purge that drew widespread condemnation and sparked concerns over possible instability.

The NIS added that since taking power, Kim has had about 70 senior officials executed.

Experts said that if Hyon's execution is confirmed, the move is aimed at strengthening Kim's control over the military.

"The purge of Hyon seems to show that it is not acceptable to challenge Kim's monolithic leadership," said Yang Moo-jin, a professor at the University of North Korean Studies. "It would be an overestimate if (a series of purges) is seen as a source of instability in the North."

   Daniel Pinkston, an analyst at the International Crisis Group, said that the purge is "the nature of authoritarian regimes," adding that it is difficult for those thinking about launching a rebellion to take collective action in the North.

"Violence always is lurking in the background as the instrument for resolving political differences," he added.

Meanwhile, the spy agency dismissed as "groundless" a report that the North's leader ordered his aunt Kim Kyong-hui to be killed via poisoning in May last year.

U.S. news cable CNN reported Tuesday that Kim, also the younger sister of late former leader Kim Jong-il, had been poisoned.

The spy agency said that there is no question about her safety, adding that she received treatment in Pyongyang in January, though currently her whereabouts are not known.

Noting that no abnormal signs within the North Korean military have been detected, Seoul's Defense Ministry said it has been closely watching situations in the North with regards to its series of saber-rattling and internal affairs.