Several media outlets quoted unnamed officials as claiming that Kim -- the man North Koreans call the "Dear Leader" -- may have recently had a stroke.
"There is reason to believe Kim Jong Il has suffered a serious health set back, possibly a stroke," an anonymous Western intelligence official told the Associated Press.
A South Korean official in Beijing told a South Korean newspaper that Kim collapsed last month.
"We have obtained intelligence that Defence Commission Chairman Kim Jong-il had collapsed on Aug. 22," the official with the South Korean embassy in Beijing told Chosun Ilbo newspaper, according to Reuters.
The same newspaper reported last week that five Chinese doctors have been visiting Pyongyang in recent weeks to treat a North Korean official, possibly Kim. There have been reports in the past that Kim suffered from diabetes and heart problems, but he has denied them.
A senior U.S. official told the AP that speculation had been circulating for weeks about the state of Kim's health and his control over North Korea's reclusive communist government.
That official said the United States has no independent confirmation that Kim is ill, but that Kim's absence at the anniversary parade appears to back up reports that he has suffered a sudden health crisis.
Kim has not missed any of the previous 10 military parades staged for major state anniversaries and other events, the New York Times reported.
The most recent health rumors began circulating in mid-August, just before North Korea announced it was suspending the disablement of its Yongbyon nuclear reactor, a key step in a landmark nuclear agreement the country had negotiated with China, Japan, South Korea, Russia and the United States.
According to media reports, North Korea's military opposes the dismantling of the reactors and the terms of the nuclear disarmament agreement were being pursued under Kim's direction.
On Aug. 26, Pyongyang's official news agency said that the country would "consider soon a step to restore the nuclear facilities in Yongbyon to their original state as strongly requested by its relevant institutions."
The U.S. administration denied any specific knowledge of Kim's health situation, though State Department spokesman Sean McCormack said there had been a decline in "outputs" from North Korea on the process surrounding denuclearization.
"We don't necessarily have a good picture into the decision-making processes of the North Korean regime, but we can see very clearly outputs or lack of outputs," he told reporters. "Over the past several weeks, we have not seen outputs in terms of their agreement to a verification regime."
Kim, who is 66 and has controlled North Korea since 1994, is known for his secretive and sometimes eccentric behavior. North Korean state media has reported that Kim has piloted jet fighters, composed operas and hit 11 holes-in-one in the first round of golf he ever played.