LONDON (CNNMoney) -- European anti-trust officials have begun investigating several major oil companies including Shell and Norway's Statoil over suspected attempts to manipulate benchmark prices.

The European Commission said its officials had carried out unannounced inspections in three European countries on Tuesday.

"The commission has concerns that the companies may have colluded in reporting distorted prices to a price reporting agency to manipulate the published prices for a number of oil and biofuel products," it said in a statement.

"Furthermore, the Commission has concerns that the companies may have prevented others from participating in the price assessment process, with a view to distorting published prices."

Oil prices assessed and published by agencies such as Platts, a unit of McGraw Hill, Petroleum Argus and Reed's ISIS serve as benchmarks for physical and derivatives markets around the world.

"We can confirm that Shell companies are currently assisting the European Commission in an enquiry into trading activities," the Ango-Dutch oil giant said in a statement. It said it was cooperating with the investigation but declined further comment.

Statoil also said it was cooperating with the investigation, which it said related to Platts' price assessment process for crude oil, refined oil products and biofuels.

"Platts confirms that the European Commission has undertaken a review at its premises in London this morning in relation to the Platts price assessment process," the agency said, adding it too was cooperating fully with the review.

The Commission carries out unannounced inspections as a preliminary step to investigate suspected anti-competitive practices.