The uncle of North Korean leader Kim Jong-un has described how fear of the deadly power politics in Pyongyang drove him and his wife to defect to the US nearly 20 years ago.

In a telephone interview with South Korea's Yonhap news agency, Ri Kang said they had been deeply concerned by what might become of them in any power struggle that followed the eventual death of Kim's father, Kim Jong-il.

Ri is married to Ko Yong-Suk, the younger sister of Kim Jong-un's mother.

"After spending nearly two decades near Kim Jong-il, I had felt the cruelty of power," Ri said on Wednesday. "I thought it wasn't such a good idea to live near that."

Ri and Ko had been tasked with looking after Kim Jong-un while he was studying in Switzerland as a teenager, and rather than return to North Korea they chose to defect to the US in 1998.