The 500-kilogram British bomb was found during construction work and police were working to defuse it Tuesday evening. One runway at the airport — Germany's busiest — was closed for landings because planes weren't allowed to fly directly over the site, news agency dpa reported.
Landings were expected to be suspended altogether for at least a short time, it said, citing air traffic control.
A stretch of the A3 highway near Offenbach, outside Frankfurt, was also closed.
Allied airplanes dropped millions of tons of ordnance on Germany during World War II. Unexploded bombs are still found frequently.