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.