Former IMF chief Dominique Strauss-Kahn was put under formal investigation by a French court on Monday over his role in a prostitution case, his lawyer said.

The move is an escalation of the case against the Socialist ex-finance minister but falls short of a formal charge.

Strauss-Kahn has denied the allegations against him, arguing that he was unaware women he met at parties organized by business associates in Lille, Paris and Washington were prostitutes.