"It will be hostile, there could be bad surprises," Forget said at the Open Sud in Montpellier, southern France.
"We have already seen in various sporting events involving Serbian fans that with a dozen idiots things could turn nasty.
"None of my players have experienced nasty atmosphere. We have to get ready for a trip to hell."
Violent Serbia fans forced their national football team's Euro 2012 qualifier in Italy to be abandoned this month, two days after angry protesters clashed with police during a gay parade in Belgrade.
Forget asked the French fans not to provoke their Serbian counterparts.
"I hope they will be discreet because the goal is not to annoy 18,000 Serbs because we know how patriotic they are," he said. "I know we will face a hostile, even maybe hateful crowd."