Kim, known in the communist North as the "Dear Leader," typically presides over mass gatherings on such occasions, waving to crowds as they shout praises to him in unison. Kim appeared at the 50th- and 55th-anniversary parades, and a 60th anniversary is considered highly important in Korean society.

His death or incapacitation would probably frustrate the U.S.-led effort to eliminate North Korea's nuclear weapons programs just as it appears to be gathering momentum. Kim was last seen in public Aug. 14; shortly after that, North Korea announced it would begin to reassemble its Yongbyon nuclear reactor, which it had agreed to dismantle under a landmark accord with the outside world.

Some U.S. officials indicated the reversal may stem from a power struggle already underway. The North Korean military is believed to have remained suspicious of the nuclear disarmament program and may now be using a Kim illness to reassert its view.

U.S. sources, citing South Korean and Japanese intelligence reports, said Kim, who is believed to be 66 or 67, suffered an apparent stroke in mid-August. One U.S. intelligence official who confirmed the apparent stroke said it possibly occurred Aug. 14, dismissing reports it happened later in August.

On Monday, North Korea's nominal No. 2 leader, Kim Yong Nam, gave a 60th-anniversary speech that referred to Kim Jong Il mainly in the past tense, said Jonathan Pollack, an Asia expert at the Naval War College. "Generally, when he is praised to the skies, it is in the present tense. But the predominant tone is looking back," he said.

Adding to the puzzle, after widespread reports that Kim failed to appear at the parade, the official Korean Central News Agency issued a short item in English: "Reception Given by Leader Kim Jong Il for Overseas Compatriots on Tuesday." No other text or photos of such a meeting were issued.

Kim has not been seen at any of the events for this week's anniversary, which included a mass games performance in a stadium that can hold 100,000 people, and a major meeting of North Korean officials, according to Kyodo News, a Japanese news agency.

North Korea, a tightly controlled, xenophobic state, has had only two leaders since its founding -- the pudgy, bouffant-haired Kim, who has ruled since 1994, and his late father, Kim Il Sung, North Korea's founder. Kim Jong Il -- who likes French wine, travels abroad only by train and presided over a famine in the 1990s that left an estimated 2 million people dead -- appears not to have named a clear successor, in contrast to his father, who had clearly identified his son as heir more than a decade before his death.

Kim has four children. His eldest son, Kim Jong Nam, 37, was caught in 2001 trying to enter Japan on a fake passport, saying he wanted to visit Tokyo Disneyland. A brother-in-law, Chang Sung Taek, briefly gained stature before being purged in 2004, but in 2006 he was rehabilitated and later named to a senior post overseeing internal security.

Kim's poor health has been a subject of speculation for decades, with experts analyzing every public appearance. The North's government treats his health as a state secret.

Charles L. "Jack" Pritchard, a former White House and State Department official who is president of the Korea Economic Institute in Washington, said there have been many false reports concerning the health of Kim and his late father over the years. But he found it significant that Kim was not spotted at the 60th-anniversary parade because of the milestone's significance in Asian cultures. "The 60th anniversary is not something to be missed," Pritchard said. "It is the fifth cycle of 12, which for Asians is a special deal."