DORTMUND, Germany (AFP) - Swiss international striker Alexander Frei is to go under the knife on Tuesday to cure a calf problem, his club Borussia Dortmund said on Friday.

Frei has not yet played this season following an operation on his hip in May but complications following that surgery have resulted in the need to operate again.

"After speaking to doctors in Switzerland and Germany, Alexander Frei decided to have an operation to cure his calf muscle problems," said a club statement.

"These problems in his right leg are a result of the operation on his hip in May."

No date has yet been set for Frei's return to action, although it will not be before the turn of the year.

"We'll talk about all that once the operation has been concluded," said Dortmund coach Thomas Doll.

The former Servette and Rennes forward scored 16 Bundesliga goals in his first season in Germany last year.