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 said on Tuesday, 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 citing a senior South Korean official with the National Intelligence Service (NIS).

2 aides allegedly executed

"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 told the briefing.

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 which regularly threatens the democratic South and the United States with destruction.

Jang, who is married to Kim'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.

Apart from domestic political problems, North Korea is involved in a protracted standoff with the West over its nuclear weapons programme.

This year, Kim Jong-un has threatened the United States with nuclear attack, declared a "state of war" with South Korea and announced he was restarting a plutonium reactor at the Soviet-era Yongbyon nuclear plant - all on top of conducting a third nuclear test in February and a long-range rocket test last December.