Nigeria's anti-graft agency has issued a sweeping indictment of several of Nigeria's 36 state governors, accusing them of widespread corruption and the looting of the public treasury.

The Economic and Financial Crimes Commission says it is investigating corruption allegations involving 31 of Nigeria's 36 state governors.

The agency says it has sufficient evidence, so far, to charge 15 governors with corruption-related offenses.

Commission Chairman Nuhu Ribadu told a tense session of the Nigerian Senate that public funds, running into hundreds of millions of dollars, have been stolen by state officials in the past seven years. He cited the case of the governor of Abia State, Orji Kalu, who - acting through friends and family - is said to have stolen about $200 million.

"We have established a prima facie case of conspiracy, stealing, corruption and abuse of office, forgery and money laundering against the governor, Orji Kalu," he said. "Governor Kalu used the following companies and enterprises belonging to himself, his mother, daughter, wife, brother in looting the Abia state treasury and building his business empire. We noted that the governor used his position in channeling states and local government funds in excess of $25 billion Naira [about $200 million]."

The governor has denied the accusation, describing it as a witch hunt.

The EFCC says formal charges will be filed against the erring governors, in October. However, state governors enjoy constitutional immunity from prosecution and can only be arrested and charged after their tenure.

Nigeria, Africa's top oil producer, is ranked among the most corrupt countries in the world.

The Catholic archbishop of Lagos, Cardinal Anthony Okogie, says endemic corruption in Nigeria is rooted in the inability of the authorities to apply existing laws in curbing criminal tendencies.

"Nigerians, we have good laws in our country, very, very good laws. Our laws can stand the test from any country, but unfortunately, we are very, very weak in the observation of these laws. Why? Because of the government," he said. "There is always a loophole, a law is made today, because I know the man at the top, and so it is bent and the young man who doesn't know the man at the top, says ooh! He has to look for ways to circumvent the law. You talk about elections, there is rigging. You talk about getting a job, there is rigging, everywhere you go to."

The crime commission has vowed to scrutinize the records of those seeking office in next April's polls, in which Nigerians are expected to elect their president and state governors, in the country's first democratic handover of power.

Several state governors have confirmed their intention to stand as candidates in the presidential ballot.