North Korea's state media was silent about his absence from the parade, a relatively low-key ceremony that unexpectedly lacked North Korea's trademark military display.
In a broadcast monitored in Seoul, Korean Central Television showed North Korea's No. 2 leader and other officials atop a viewing stand. Kim Jong Il was not shown.
In Washington, a Western intelligence official said there is reason to believe the 66-year-old, known to his people as the "Dear Leader," was gravely ill after he failed to show up at the celebration.
"There is reason to believe Kim Jong Il has suffered a serious health setback, possibly a stroke," the official said.
Another official said rumors and reports of a possible illness were based in part on intelligence gathered by other nations.
At her White House briefing, press secretary Dana Perino initially said she had no information on Kim's health. "I have nothing to give you today," she told journalists, CBS News White House correspondent Mark Knoller reported.
A senior U.S. official said rumors had been circulating for weeks about 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.
Japan's Kyodo news agency reported that Kim did not attend and North Korea's state news agency has made no mention of Kim appearing in public Tuesday.
Kim's last appearance reported by North Korean media came on Aug. 14. South Korean media have reported in recent days that he may be ill and receiving medical treatment, citing government officials.
The South Korean government says it has been unable to confirm them.
Kim's health has been a focus of intense interest because his fate is believed to be closely tied to that of the secretive totalitarian state that he inherited in 1994 from his father in communism's first hereditary transfer of power.
Kim Jong Il took over the communist country after Kim Il Sung died of heart failure - a death that was not announced for 34 hours. He has three sons with two different mothers but has not anointed any of them as his successor.
A spokesman for South Korea's main spy agency, the National Intelligence Service, said it could not immediately confirm Kim's absence. The rally involved about 1 million people, the spokesman said, on condition of anonymity, citing office policy.
The centerpiece of the celebration had been expected to be a massive military parade through Pyongyang's central Kim Il Sung Square - named after the communist country's founding figure - as normally happens in key anniversary years.
The footage broadcast on North Korean television showed what it described as civilian militia goose-stepping through the square.
Kim Jong Il attended the parade on the 50th and 55th anniversaries.
South Korean media have speculated that the 66-year-old Kim's health has worsened. South Korea's intelligence service has previously said Kim has chronic heart disease and diabetes - denied by Kim himself.
South Korea's Chosun Ilbo newspaper reported Tuesday that Kim collapsed on Aug. 22, citing an unnamed South Korean diplomat in Beijing. The diplomat got the information from a Chinese source, the paper said.
A Japanese scholar and expert on North Korea, Toshimitsu Shigemura, a professor at Tokyo's Waseda University, has even claimed recently that Kim actually died in 2003 and that the North has been using body doubles of Kim for public events.
Chinese Foreign Ministry spokeswoman Jiang Yu said at a regular news briefing Tuesday that she had no information about Kim's health and had not heard that he was ill. She did not answer a question asking when the last time was that Chinese officials spoke directly with Kim.
The North's 60th anniversary comes amid international doubts over its commitment to denuclearization, speculation about the health of its leader and a worsening food crisis.
North Korea's state news agency had made no mention of the parade late Tuesday, though it carried an exhortation from the main Rodong Sinmun newspaper calling on the population to remain united around Kim.
It also called for a stronger military, describing the armed forces as "the foundation of a strong nation."
South Korea said last week the North has begun restoring its atomic facilities in apparent anger over not being removed from a U.S. list of countries that sponsor terrorism.
North Korea - which conducted an underground nuclear test blast in October 2006 - began disabling its main nuclear facilities late last year in exchange for international energy aid and other benefits.