Chinese sources close to North Korean affairs say the wife of North Korean leader Kim Jong-il, Ko Yong Hee, died of a heart attack on August 13th.

The sources said they had confirmed Ko's death through a number of channels and that a funeral service has already been held in the North.

There had been persistent speculation that the 51-year-old Ko may have died following breast cancer-related surgery in France earlier this year, but this is the first public confirmation of her death.

The sources said it is tradition for the North to keep secret anything that has to do with the private life of the North Korean leader and his family due to fears over potential power struggles.

The late Ko had two sons with Kim, Jong-chol and Jong-un, who was reportedly being groomed as heir-apparent.

The North Korean leader has another son, Jong-nam, by his first wife Sung Hae-rim, but it is his second wife, Ko, who has been referred to as the "respected mother" and "great woman" by the North's state media in recent years.