South Korean officials said Thursday they were investigating reports that the woman considered to be North Korean leader Kim Jong Il's most influential wife has died after a long battle with breast cancer.

News that Ko Young Hee -- idolized in North Korea as the nation's "revered mother" -- apparently succumbed to her illness was first reported Wednesday on the Web site of an investigative journalist for Chosun, a monthly South Korean magazine based in Seoul. South Korea's government-owned KBS Television followed up Thursday with a similar report, citing unnamed diplomatic sources in Beijing. Officials in Seoul said they were still trying to confirm the death.

Ko, 52, has been viewed as the foremost of at least three women considered to be among Kim's wives or consorts, though it remains unclear whether he officially married any of them. Little is known about Ko, but details about her life were provided by U.S. and South Korean intelligence analysts who specialize in North Korea.

She was believed to have been suffering from breast cancer for several years, and her death was widely expected in intelligence circles following her return to North Korea after a reported hospitalization in Paris in April. Unconfirmed reports in the South Korean media indicated that the North Koreans have ordered an expensive coffin custom-made in France.

Information on Kim and his family is closely guarded in North Korea, and the official press in Pyongyang, the capital, did not mention the reports.

Analysts said Ko's death, if confirmed, could affect the selection of a successor to Kim, 62, who inherited his post from his father, Kim Il Sung, in the first succession by bloodline in a Communist nation. Kim Il Sung, North Korea's founder, died in 1994.

Intelligence officials said they believed Kim Jong Il's three sons -- two of them mothered by Ko -- are in the running to succeed him.

In a country where Kim rules in part through claims of divinity, Ko became the subject of a glorification campaign beginning in the summer of 2002 and had been using her status and influence with Kim and the military to ensure that one of her sons -- Kim Jong Chul, 23, and Kim Jong Woong, 19 -- was picked as their father's successor.

During the campaign to elevate Ko's stature, the North Korean military has been celebrating the former professional dancer with lofty slogans and songs. Any slight to her name is considered a high crime in North Korea.

Kim's third son is Kim Jong Nam, 33, whose mother, Sung Hae Rim, died in a Moscow hospital in 2002. Kim Jong Nam, who as Kim's eldest son would traditionally be first in line of succession, is said to have had a stormy relationship with Ko.

A year earlier, Kim Jong Nam was arrested in Japan for using a Dominican passport in an attempt to visit Tokyo Disneyland -- a disgrace that analysts said forced him into a period of exile in China, Southeast Asia and Europe and may have cost him the top job in Pyongyang.

However, U.S. and South Korean intelligence officials said they believed Kim Jong Nam had returned to Pyongyang and was apparently taking a lead role in running North Korea's secret police.

Some experts said the campaign to deify Ko while she was ill may signal that Kim has decided to name one of her sons as the next leader. But others argued that her death may help Kim Jong Nam persuade his father to anoint him.

Ko, the daughter of a Korean immigrant in Japan, moved to North Korea in the 1960s. She has been considered one of her husband's closest confidants, and analysts voiced concern that her death would upset the North Korean leader at a critical time.

Kim is in the midst of a high-stakes standoff with Washington and neighboring nations over his highly developed nuclear weapons programs.

On Thursday, South Korean Deputy Foreign Minister Lee Soo Hyuck said in a speech in Seoul that serious progress in the six-party nuclear talks was unlikely until after the U.S. presidential election.

"Ko was an important person, and if it's true that she has died, then there will be some degree of impact," said Osamu Eya, a Tokyo-based North Korea specialist. "But there won't be major changes. Kim Jong Il will continue to rule with an iron fist."

Cho reported from Seoul.