"The information behind the threat is very real," a senior US official told ABC News.
"The danger level is high. We are part of the global threat by Islamist terrorism," said German Interior Minister Wolfgang Schäuble.
ABC said officials are particularly worried about the military facility called Patch Barracks near Stuttgart, where the US European Command is headquartered.
Security has been beefed up at all US military and diplomatic facilities following reports that suspected terrorists had had Patch Barracks under surveillance.
"The attack would be designed to create high numbers of casualties among both Germans and the US military," Richard Clarke, a former White House counter-terror official told ABC.
A senior unnamed US official was quoted by the CNN network as saying that the group was believed to be an affiliate of the al Qaeda terror network, and was thought to have bombs and small arms.
"There are 300 to 500 people who are suspected to be part of al Qaeda cells in Germany," retired colonel Andrews Pratt of the George Marshall Center in Germany told ABC.
"In a democratic state like Germany, you just can't go out and arbitrarily arrest people because they are under suspicion," Pratt said.
Report comes in charged atmosphere
The September 11 hijackers planned their operation out of Hamburg, Germany, ABC recalled.
Mounir el Motassadeq, a Moroccan accomplice in the September 11, 2001 was sentenced in January by a German court to 15 years in jail for his role in the death of 246 passengers and crew on the four hijacked aircraft used in the attacks.
Motassadeq, the first person ever convicted for complicity in the attacks on the World Trade Center and the Pentagon, has admitted having links to the hijackers but has maintained his innocence during a long battle with German courts.
The report comes after US authorities said this week they had foiled an alleged plot to attack the Fort Dix military base in the US state of New Jersey, arresting six foreign-born men in the case.
Germany is to host a meeting of leaders of the Group of Eight most industrialized nations in the Baltic Sea resort of Heiligendamm on June 6-8.