A watchdog group says four African countries lost more than $4 billion in shady oil and mining deals and calls for the Extractive Industries Transparency Initiative to police the hidden ownership of companies used for such corruption.

Global Witness details contracts in Nigeria, Angola, Republic of Congo and the Democratic Republic of Congo and "lays bare how anonymous companies facilitate the loss of national wealth on an epic scale."

The report questions the role of international oil companies that facilitate such deals.

Global Witness says the transparency group faces "a credibility test" when its board meets in Switzerland this week to decide whether to make disclosure of true owners a membership condition.

Nigeria and both Congos belong to the transparency body while Angola is trying to join the 48-nation organization.