"It will be a bloody battle that will leave deep scars in the balance sheets," Ulrich Homburg, director of passenger transport for Deutsche Bahn told the Financial Times Deutschland.
SNCF plans to offer inter-city service in Germany, either under its own name or that of its public transport subsidiary Keolis, a source close to the matter told AFP on Thursday.
The project is likely to complicate already tense relations between the two state-owned railways.
Deutsche Bahn has complained several times it is prevented from competing in France while SNCF already runs local trains in Germany via Keolis.
"In war there is no winner," Homburg said in a warning to the French railway.
He also called on Chancellor Angela Merkel to defend Germany's interests.
Another Bahn director, who was was not identified by the newspaper, said cross border cooperation between the two railways on high-speed service could suffer as a result of SNCF's plans.