Media playback is unsupported on your device Media caption State media announced the execution of Chang Song-thaek

The once-powerful uncle of North Korean leader Kim Jong-un has been executed after being purged for "acts of treachery", state media say.

Chang Song-thaek was dramatically removed from a special party session by armed guards earlier this week.

It was the biggest upheaval since Mr Kim succeeded his father two years ago.

State news agency KCNA said Mr Chang had admitted at a military trial on Thursday to attempting to overthrow the state, and was executed immediately.

Mr Chang, who is thought to have mentored his nephew during the leadership transition from Kim Jong-il to his son Kim Jong-un in 2011, was "despicable human scum... worse than a dog", said the agency.

Image caption An image of Mr Chang in court, his hands bound, was released on Friday

Image caption Before his fall from grace, he was frequently pictured beside leader Kim Jong-un

Image caption Commuters in Pyongyang crowd around a public newspaper to read about the execution

He had admitted abusing his positions of responsibility to form a faction against the state and to harbouring his own political ambitions, it said in a lengthy and detailed report.

In the US, the White House said it could not independently verify reports of the execution but had "no reason to doubt" them.

Chang Song-thaek 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: Dismissed from his position

December 2013: Executed as a "traitor" Profile: Chang Song-thaek Secretive 'first family' What is known about N Korea's purge?

"If confirmed, this is another example of the extreme brutality of the North Korean regime. We are following developments in North Korea closely and consulting with our allies and partners in the region," it said in a statement.

China, North Korea's main economic backer, said the execution was an "internal matter" for North Korea.

However, the BBC's Martin Patience in Beijing says the bland statement is likely to mask deep concern and will raise questions as to how much influence China actually has over the nuclear-armed state.

Mr Chang - married to the elder Kim's sister - had held senior posts in the ruling party and was vice-chairman of the powerful National Defence Commission, the North's top military body.

He was frequently pictured alongside his nephew and seen by some observers as the power behind the throne.

But in early December, it emerged that he had been removed from his senior military position and that two of his aides had been executed.

Then on Monday, KCNA broadcast footage of him being removed from a party session by uniformed guards.

In a long report on Friday, KCNA said: "Chang dreamed such a foolish dream that once he seizes power by a base method, his despicable true colours as 'reformist' known to the outside world would help his 'new government' get 'recognised' by foreign countries in a short span of time."

It also said Mr Chang:

Attempted to "overthrow the state"

Transformed his department into "a 'little kingdom'" and attempted to "trigger off discontent" within the army to mobilise a coup

Took control of the "major economic fields of the country" and "schemed to drive the economy of the country and people's living into an uncontrollable catastrophe"

Committed corruption by transferring construction units to his contacts

Committed irregularities related to a joint economic zone set up with China

Was responsible for unpopular currency reforms in 2009

Under questioning Mr Chang is alleged to have confessed: "I was going to stage the coup by using high-ranking army officers who had close ties with me or by mobilising armed forces under the control of my confidantes.

Analysis This purge is striking for its speed and publicity. The crimes Chang Song-thaek was accused of - disloyalty and insurrection - are the most serious North Korea can muster. His proximity to power and his positions of responsibility had, state media said, led him to see himself as equal to the country's young leader, and seek to topple him. Chang Song-thaek had a lot of power over economic decisions, and regularly handled relations with China. Some believe that Chang's admiration for China's growth model led to policy differences within the regime. But others say this is a battle for control of the North Korean state; that its young leader, Kim Jong-un, had outgrown his uncle's protection, and saw him instead as a rival and potential threat. Chang Song-thaek has already been edited out of official documentaries: his story rewritten by the country's powerful propaganda machine. His dramatic fall from grace, designed to hammer the message home: that no one - not family members, not North Korea's most senior figures - is beyond retribution. But the worry remains: how much does this story reveal about instability at the heart of the North Korean regime?

"I didn't fix the definite time for the coup. But it was my intention to concentrate my department and all economic organs on the cabinet and become premier when the economy goes totally bankrupt and the state is on the verge of collapse."

There was no immediate word about the whereabouts of Mr Chang's wife, Kim Kyung Hee.

Analysts say Mr Chang's fall from grace could be seen as the latest in a series of carefully calibrated moves to demonstrate Kim Jong-un's authority and an assertion of his independence.

In August 2012, Mr Chang made a high profile trip to China, where he met then-President Hu Jintao. The two sides later signed a raft of economic deals, including the development of two special economic zones: Rason, on North Korea's east coast, and Hwanggumphyong, on the border with China.

The BBC's Lucy Williamson in Seoul says one theory about Mr Chang's demise is that his work with China had led him to admire some of Beijing's economic reforms.

But it is more likely that he presented a perceived threat to his nephew's authority, she says.

Professor Lee Jung-hoon, from South Korea's Yonsei University, told the BBC that the move showed that North Korea was "very unstable".

"[For Kim Jong-un] to go to the extent to actually purge him and execute him says a lot about the state of things in that country," he said.

Media playback is unsupported on your device Media caption State TV in North Korea announced the execution of a "wicked political careerist"

As news of the purge emerged earlier this week, South Korean President Park Geun-hye warned the North was "carrying out a reign of terror" to reinforce Mr Kim's position.

On Friday, South Korea's military said it had tightened surveillance on Pyongyang, news agency Yonhap reported.

Meanwhile, Japan's top government spokesman Yoshihide Suga said: "We will calmly monitor the situation while communicating with other countries and collect relevant information."