Image copyright Reuters Image caption Many observers saw Mr Chang as having a major influence on Kim Jong-un

A powerful uncle of North Korean leader Kim Jong-un has been removed from his post, South Korean media reports say.

Citing South Korea's intelligence agency, they say Chang Song-thaek, 67, lost his position as vice-chairman of the North's top military body.

Two close aides were also executed for corruption, according to the reports.

If confirmed, Mr Chang's removal would be the biggest upheaval in North Korea's leadership since Mr Kim succeeded his father, analysts say.

Analysis The reports attributed to South Korea's spy agency are difficult to verify. The agency has a patchy record in predicting North Korea's internal changes, and there is little scope to assess the evidence behind the claim. Chang Song-thaek is Kim Jong-un's uncle by marriage. As such, his influence over the leadership is independent from his senior military and political positions. Mr Chang's prestige has increased since the 2011 transition. He is seen as an economic reformer, and is regularly tasked with handling talks with China, Pyongyang's only major ally. His removal, if true, would be a dramatic change, but much would also depend on the reason for his dismissal. If medical problems led to his temporary disappearance from the limelight, this might leave him with great unofficial influence. Political differences or disloyalty would signal a far more serious shift. Either way, like many reports about North Korea's internal politics, the truth of this one may take a long time to emerge.

Kim Jong-un took over after Kim Jong-il died in 2011.

The latest reports emerged from an intelligence briefing given to South Korean lawmakers.

The National Intelligence Service (NIS) made the assessment based on information provided by multiple sources, the South's Yonhap news agency said.

It also quoted the intelligence service as saying two of Mr Chang's closest associates had been executed in public in late November.

'Power behind Kim'

The BBC's Lucy Williamson in Seoul says the reports are difficult to verify, and South Korea's spy agency has been proven wrong before.

But if true, the development would mark a significant shift, she adds.

Mr Chang is married to Kim Jong-il's sister.

He has often been pictured beside the elder Kim's son Kim Jong-un and was seen by some observers as the power behind him.

Mr Chang climbed through the ranks of the secretive leadership of North Korea's Korean Workers' Party (KWP) in the 1970s.

In 1992, he was elected to the party's Central Committee. But, despite his senior status, he has been targeted by purges in the past.

Chang Song-thaek Image copyright Reuters Born 1946; marries Kim Jong-il's sister in 1972

Joins Korean Workers' Party administrative ranks in 1970s

Elected to Central Committee in 1992

Sidelined in 2004, but rehabilitated in 2006

2011: Gets top military post under Kim Jong-un

Nov 2013: Reportedly dismissed Profile: Chang Song-taek Secretive 'first family'

In 2004, despite his place in the Kim family, he disappeared from public view.

One report at the time, citing South Korean intelligence, said Mr Chang had been placed under house arrest.

Others suggested he had been sent for "re-education". However, two years later he appeared to have been reinstated.

He was regarded as an economic reformer and a major influence on Kim Jong-un.

He held key positions in both the KWP and the National Defence Commission.

Apart from chronic economic problem, North Korea is involved in a protracted stand-off with its neighbours and Western powers over its nuclear weapons programme.

Tensions between the two Koreas rose after the North's third nuclear test in February.

Angered by expanded UN sanctions and annual US-South Korea military drills, Pyongyang threatened attacks on Japanese, South Korean and US military targets in the region.