Former IMF head Dominique Strauss-Kahn is being questioned by French police as a suspect in a prostitution ring inquiry, a prosecutor says.Mr Strauss-Kahn, once a front-runner for the French presidency, is being held in custody at a police station in Lille, northern France.Police have questioned a number of prostitutes who have admitted having sex with Mr Strauss-Kahn.He insists he did not know that the women were prostitutes."I challenge you to distinguish a naked prostitute from any other naked woman,” his lawyer has told French television.Mr Strauss-Kahn resigned as head of the International Monetary Fund (IMF) in May 2011 after he was charged in New York with the attempted rape of a hotel maid. The case was later dropped.In this separate inquiry, French police have already placed two men under preliminary investigation on charges of organising a prostitution ring and misusing corporate funds to pay for sex.Mr Strauss-Kahn is said to have taken part in sex parties in Paris and Washington in late 2010 and early 2011.He could be held by police for up to 48 hours, French media say. Although consorting with prostitutes is legal in France, supplying them to others is not.