Quoting "multiple sources", a report from South Korean news agency Yonhap says the relatives include North Korea's ambassadors to Cuba and Malaysia, who were both related to Mr Kim's uncle by marriage.

Mr Jang, once a powerful mentor to Mr Kim, was dragged out of a party meeting, tried and executed last month on charges of attempting to overthrow the communist regime.

The report says all direct relatives of Mr Jang have been executed.

"Extensive executions have been carried out for relatives of Jang Song-thaek," one source told the agency.

"All relatives of Jang have been put to death, including even children."

The agency says those killed include Mr Jang's sister Jang Kye-sun, her husband and ambassador to Cuba, Jon Yong-jin, and the ambassador to Malaysia, Jang Yong-chol, who is a nephew of Mr Jang. The nephew's two sons were also killed, according to the report.

Yonhap says all the relatives were recalled to Pyongyang in early December and executed. The dead include all the sons, daughters and grandchildren of Mr Jang's two brothers.

"Some relatives were shot to death by pistol in front of other people if they resisted while being dragged out of their apartment homes," a source told Yonhap.

The agency says the regime did spare some of the family. They have been sent to remote villages.

The report says Mr Kim is also carrying out a purge of low-level officials linked to Mr Jang.

Before falling out with Mr Kim, Mr Jang was considered the second-most powerful man in North Korea.

Married to the sister of Mr Kim's father, Kim Jong-il, Mr Jang had been considered the man who could help his nephew establish himself in power but at the same time presented the greatest threat to the young and untested leader.

Mr Jang's widow, Kim Kyong-hui, was thought to have survived the purge, but was later reported to have either died of a heart attack or taken her own life.

South Korean news agencies have been known to publish unverified stories emanating from the North, but the most recent claims tie in with a report by the Daily NK news site, run by North Korean defectors based in South Korea, which claimed more than 100 relatives of Mr Jang had been rounded up  before Christmas.

SMH