Doubts surfaced Thursday over the reported execution of North Korea's defence chief as South Korea's spy agency clarified that it had been unable to verify he had been put to death.

Briefing a select parliamentary committee on Wednesday, the National Intelligence Service said Defence Minister Hyon Yong-Chol had been purged, and cited intelligence suggesting he may have been executed on April 30 using an anti-aircraft gun.

The grim details of Hyon's demise dominated headlines, but the NIS on Thursday stressed that his execution had never been confirmed.

"Hyon has been purged," an NIS spokesman told AFP.

"And there are intelligence reports that he might have been executed, but this has not yet been verified," the spokesman said.

The confusion is partly the result of the way NIS briefings to parliament are carried out and reported.

They take place behind closed doors, after which selected lawmakers pass on the information to the South Korean media -- resulting in a gap between the original NIS briefing and the resulting headlines.

According to the lawmakers, the NIS said Hyon was purged for disloyalty and dozing off during official events presided over by leader Kim Jong-Un.

If confirmed, it marks the most high-profile elimination of a top Pyongyang official since the purge and execution of Kim's powerful uncle, Jang Song-Thaek, in December 2013.

The NIS had correctly reported Jang's downfall before it was confirmed by North Korea, but the agency has had some intelligence failures as well as successes in analysing events north of the border.

- Questionable conclusions -

The NIS knows its briefings will be relayed to the media, so the information it divulges is usually deemed to have been carefully vetted for accuracy.

But some of the lawmakers who attended Wednesday's briefing, as well as some independent analysts, have questioned the agency's conclusion on Hyon.

Shin Kyoung-Min, an opposition MP, said it was "odd" that North Korean state TV had continued to show recorded footage featuring the defence minister even after he had supposedly been purged.

The state-run media typically deletes all past mentions of purged officials and air-brushes them from any TV footage.

Such officials are often simply not heard of again, and their fate could range anywhere from demotion and exile to the countryside, to jail or execution.

But old TV video of Hyon accompanying Kim was broadcast as recently as May 12.

"If Hyon has really been purged, and even executed, the TV station wouldn't be making such a mistake," Shin told a local radio station.

A search of Hyon's name on the website of Rodong Sinmun, the newspaper of the North's ruling Workers' Party, produced dozens of news stories -- the most recent dated April 30.

The same search on the North's official website, Uriminzokkiri, also produced hundreds of news stories.

The April 30 article in Rodong Sinmun named Hyon among those officials attending musical performances on April 28 and 29.

Cheong Seong-Chang, an analyst at the Sejong Institute think-tank in Seoul, said it was "highly unlikely" the North had arrested Hyon only a day later and executed him immediately.

"It's not like he was attempting to assassinate Kim Jong-Un ... so it is really hard to believe that he was executed in such a hasty manner," Cheong said.

"Also, his name should have been deleted from the North's websites long ago if he was really executed ... like Jang Song-Thaek," Cheong said.

All images and mentions of Jang, who often accompanied Kim on "field guidance trips" across the country, were deleted from official records after he was purged.

A current search of Jang's name on both Uriminzokkiri and Rodong Sinmun only shows stories relating to his arrest, purge and execution.