The incident allegedly happened during a sex party in a Washington hotel while Strauss-Kahn was head of the International Monetary Fund. Strauss-Kahn says whatever happened was consensual and he denied doing anything violent.
However two Belgian prostitutes said in the French daily Liberation this month Strauss-Kahn was violent during sex at the W Hotel in Washington. One claimed he forced a sexual act on her despite her protests.
The investigation into the accusations is part of a suspected prostitution probe in France.
Strauss-Kahn was once seen as a contender for France's presidency. His reputation has crumbled after he was accused of attacking a New York hotel maid last year. Those charges were later dropped, but despite claims of diplomatic immunity, a judge ruled the case will move forward.
The Associated Press contributed to this report.
