There was no sign of Kim at a closely watched parade Tuesday marking the 60th anniversary of North Korea's founding, and the country's state media was silent about his absence. His last reported public appearance was August 12.
"There is reason to believe Kim Jong Il has suffered a serious health set back, possibly a stroke," a Western intelligence official said, speaking on condition of anonymity.
Another official said rumors and reports of a possible health setback were based in part on intelligence gathered by other nations.
A senior U.S. official said rumors had been circulating for weeks about the state of Kim's health and his control over North Korea's highly centralized government.
That official said the United States has no independent confirmation that Kim is ill, but that Kim's absence lends credence to reports that he is suffering and may no longer be in a position to command the absolute authority he had wielded.
"What we do know is that he was not at the military parade," the official said, speaking on condition of anonymity about the administration's internal assessment of the situation. "That is quite unusual and reinforces a lot of what we've been hearing."
All the officials spoke on condition of anonymity to describe sensitive intelligence gathering.
In a broadcast monitored in Seoul, Korean Central Television showed North Korea's No. 2 leader and other officials atop a viewing stand on Tuesday. Kim was not shown.
The rumors began circulating in mid-August. Shortly thereafter, North Korea announced it was suspending the disablement of its Yongbyon nuclear reactor, a key step in abandoning its atomic weapons program that it agreed to take in negotiations with China, Japan, South Korea, Russia and the United States.
North Korea's powerful military is known to have been opposed to the measure, and many analysts believed it was proceeding mainly due to Kim's support for the process and the backing of moderates in his foreign ministry.
When North Korean diplomats notified their U.S. counterparts of the suspension, they did so in a note that explained the move was being taken "due to pressure from the relevant agencies," according to people familiar with its contents.
Kim has held absolute power in the Stalinist regime, and the wording of the note set off alarms that his control over the disarmament scheme may be in question.