North Korean leader Kim Jong Un's uncle, considered the power behind the throne, is believed to have been dismissed from his posts, a South Korean lawmaker has said, suggesting a huge upheaval in one of the world's most secretive states.

Jang Song Thaek was likely sacked as vice chairman of the North's powerful National Defence Commission and as a department head of the ruling Workers' Party, the lawmaker, Jung Cheong-rae, said on Tuesday, citing a senior South Korean official with the National Intelligence Service (NIS).

"The briefing by an NIS senior official was that they believe Jang Song Thaek has lost his posts," Jung told a news briefing.

Two close aides to Jang in the Workers' Party had also been executed for corruption, Jung said, also citing the briefing.

"Following that, the NIS said it believes Jang Song Thaek has not been seen and has lost his posts," Jung added.

There was no immediate mention of Jang's fate on North Korea's KCNA news agency, the primary source of information on the country for outsiders.

Jang, who is married to Kim Jong Un's aunt, Kyong Hui, has been the central figure in a coterie of top officials and family members who worked to ensure the young and untested son of Kim Jong Il took over power when his father died in 2011.

Jang, who is widely seen as an advocate of economic reform, was previously purged in a power struggle in 2004 under Kim Jong Il's rule but was reinstated two years later.

Analysts who watch the North's power structure say Jang's removal would not have been possible without leader Kim Jong Un's approval.

Source: Agencies