South Korea's largest daily newspaper, the Chosun Ilbo, said Kim, who is 66-years-old and believed to be suffering chronic illness, collapsed last month, citing a South Korean diplomatic source in Beijing.
Kim attended the parades for the 50th and 55th anniversary of the state founded by his father, Kim Il-sung.
Chosun reported on Saturday that five Chinese doctors had been in North Korea for more than a week, possibly to treat Kim.
Tuesday's parade featured displays of armaments, legions of goose-stepping soldiers and tens of thousands of North Koreans shouting praises to Kim in unison, according to a North Korea state TV broadcast monitored in Seoul.
Previous anniversaries have been marked by thousands of troops and military hardware parading through Kim Il-sung square in the centre of the North Korean capital.
The square is dedicated to the nation's "eternal president" and Kim Jong-il.
|Kim is rarely absent from major military parades [AFP]
Tuesday's celebrations come amid renewed international concern over North Korea's nuclear programme, following reports that it has taken steps to restart its main plutonium-producing reactor.
North Korea began taking apart its Soviet-era Yongbyon nuclear plant in November under the terms of a five nation disarmament-for-aid deal brokered by China.
The North, which is believed to have tested a nuclear device about two years ago, had completed most of the required disablement steps, but stopped the process in August in protest at what it said was Washington's failure to drop it from a US "terrorism" blacklist.
Washington has said the North must first agree on a system to verify disclosures about its nuclear programmes.