The aunt of North Korean leader Kim Jong-un appeared to have returned to Pyongyang last month after receiving treatment abroad for heart disease, a diplomatic source with knowledge in the matter said Friday.

The whereabouts of Kim Kyong-hui, a younger sister of late North Korean leader Kim Jong-il, the father of the current leader, have been a focus of media attention following last week's stunning purge and execution of her husband, Jang Song-thaek. 

"To my knowledge, Kim Kyong-hui had been receiving treatment for heart disease and returned to North Korea last month," the source said on the condition of anonymity.

Where precisely she had been treated last month, whether it was China or Russia, is unclear, the source said.

Despite the dramatic downfall of her husband, she appears to be safe as she was named to a list of top officials for a public function -- state funeral for senior Workers' Party official Kim Kuk-thae, who died on Dec. 13. However, she didn't appear at the second death anniversary of the late North Korean leader earlier this week.

Kim Jong-il died of heart attack on Dec. 17, 2011.

The execution of Jang, who was one of key regents for securing the succession of the younger Kim, raised questions about the stability of Kim's two-year-old leadership.  (Yonhap News)