Nigerian lawmakers say their country's oil business must be investigated after an international watchdog group alleged that Nigerian and Swiss traders may be complicit in defrauding the Nigerian public of billions of dollars.

Nigeria's parliament ordered an inquiry into oil trading following the report last week by the Swiss-based group, Berne Declaration, which calls Geneva a "haven for Nigerian fraudsters" and accuses Swiss traders of possibly being involved in "one of the most massive frauds that the African continent has experienced."

Most of Nigeria's national budget comes from oil revenue and lawmakers last year said that between 2009 and 2011 Nigerian oil officials and their partners stole $6.8 billion intended to subsidize the price of fuel.

Nigerian oil officials and the Swiss companies deny the corruption charges in the report.