US and German officials believe Islamic terrorists are in advanced stages of planning an attack on American military personnel or tourists in Germany, a television channel reported on Friday.
Quoting law enforcement officials, ABC news said US air marshals have been diverted to provide expanded protection of flights between Germany and the US.
"The information behind the threat is very real," a senior US official told the channel.
German Interior Minister Wolfgang Schauble told reporters that "the danger level is high. We are part of the global threat by Islamic terrorism."
Officials are particularly worried about a military facility near Stuttgart called Patch Barracks, headquarters of the US European Command.
Security at all US military and diplomatic facilities have been increased in the past month following reports that suspected terrorists had conducted surveillance of the Patch Barracks facility, ABC said.
"The attack would be designed to create high numbers of casualties among both Germans and the US military," said ABC news consultant Richard Clarke, a former White House counter-terror official.
The 9/11 hijackers planned their operation out of Hamburg, Germany, and the country continues to be known as a staging area for Al Qaeda and groups affiliated with it.