It was not clear how the NIS received the information and it is not possible to verify such reports from within secretive North Korea independently.
Hyon became one of Pyongyang's most senior officials last June when he took up his ministerial position and was soon appointed to the National Defence Commission, the country's most powerful organ. His name regularly appeared near the top of the list of officials accompanying supreme leader Kim Jong Un to public events.
A rise in executions has been interpreted in Seoul as part of efforts by Mr Kim to protect his authority over the state since taking power on his father's death in December 2011.
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Shin Kyung-min, a lawmaker, cited NIS claims that 15 officials had been killed so far this year, compared with 41 last year and 10 the year before. "Kim seems to be maintaining a "Reign of Terror" strategy, executing people . . . when he feels threatened," he said.
In 2013 Mr Kim's uncle and top adviser was purged and executed, along with several other senior officials.
Hyon's apparent downfall came less than a month after he attended a conference in Moscow where he condemned the presence of US forces in South Korea as a "cancer growth". In September last year he represented the country in a meeting with Russia's President Vladimir Putin.
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A report by Hong Kong's Phoenix TV this month claimed that Hyon had made a failed attempt to buy missiles and launch equipment from Russia during his latest visit, prompting Mr Kim to stay away from a second world war commemorative event in Moscow on May 9.
The NIS claim that an anti-aircraft gun was used in the execution follows a report last month by AllSource Analysis and the Committee for Human Rights in North Korea, which found satellite images from last October that appeared to show execution victims lined up in front of a battery of six anti-aircraft guns.
- Additional reporting by Reuters