The US Justice Department is conducting a criminal inquiry of nearly a dozen oil and oil-services companies, focusing on potentially illegal payments to Nigerian Customs agents through allegedly Panalpina World Transport Holding, a Swiss-based shipping and logistics company.

Eleven oil and oil-service firms received a letter from the Justice Department asking them to detail their relationship with Panalpina.

The oil and oil-service firms were asked to list the countries where Panalpina provided it with services in the past five years, and to specify what it paid for those services.

Panalpina announced that it was conducting an internal investigation and has been asked to provide documents to the Justice Department relating to services in Nigeria, Kazakhstan and Saudi Arabia for "a limited number of customers".

"Obviously, we are co-operating with the investigation," Panalpina spokesman Martin Spohn said.

The Justice Department probe underscores the difficulty of complying with US anti-bribery laws in countries that may be as awash in corruption as they are in oil.

Bribery is rampant in Nigeria. "It's a very difficult operating environment, even if you're trying to do everything by the book," said an individual familiar with operating in Nigeria.

Another individual, who has conducted business in Nigeria, said corruption in Nigeria is so widespread, "you can't pass through the Lagos airport without being asked to pay a bribe".

Firms that refuse to pay bribes cannot obtain permits needed to conduct business in Nigeria, while those that pay run the risk of being charged with violating the US Foreign Corrupt Practice Act.