The warning, posted on October 12, was part of an ongoing internet forum titled “New Attack on America Be Afraid”. It mentioned football stadiums in New York, Miami, Atlanta, Seattle, Houston, Oakland and Cleveland, where games are scheduled for this weekend.
The Homeland Security Department alerted authorities and stadium owners in those cities, as well as the NFL, about the web message but said the threat was being viewed "with strong scepticism."
Homeland Security spokesman Russ Knocke said there was no intelligence that indicated such an attack was imminent, and he said the alert was "out of an abundance of caution."
"The department strongly encourages the public to continue to go about their plans, including attending events that involve large public gatherings such as football games," Mr Knocke said.
The FBI also expressed doubt about the threat.
The nation's alert level remains at yellow, signalling an elevated risk of an attack. The threat level for airline flights is at orange, a higher level, where it has been since a foiled plot to bomb US-bound commercial jets was revealed on August 10.
NFL spokesman Greg Aiello said stadiums around the country "are very well protected through the comprehensive security procedures we have in place, including secure facility perimeters, pat-downs and bag searches."
Officials were made aware of the web posting on October 16. The threat was timed to be carried out on Sunday, October 22, marking the final day in Mecca of Ramadan, the Islamic holy month.
"The death toll will approach 100,000 from the initial blasts and countless other fatalities will later occur as result from radioactive fallout," according to a copy of the posting.
The bombs, according to the posting, would be delivered to the stadiums in trucks. All but one of the stadiums - Atlanta - are open-air arenas, the posting noted, adding: "Due to the open air, the radiological fallout will destroy those not killed in the initial explosion."
Explosions would be nearly simultaneous, the posting said, with the cities specifically chosen in different time zones.
The posting said that al-Qaeda would automatically be blamed for the attacks and predicted, "Later, through al-Jazeera, Osama bin Laden will issue a video message claiming responsibility for what he dubbed 'America's Hiroshima."'