European anti-trust officials carried out Tuesday spot raids on the gas sector, over fears companies breached rules to protect market competition and consumers, the European Commission said.
European Commission officials "undertook unannounced inspections at the premises of companies active in the supply, transmission and storage of natural gas in several member states", said a statement.
"The commission has concerns that the companies concerned may have engaged in anti-competitive practices in breach of EU anti-trust rules or that they are in possession of information relating to such practices," it added
The raids took place on premises in central and eastern European countries, with an investigation focused on the supply of gas upstream "where, unilaterally or through agreements, competition may be hampered or delayed".
The commission did not name the companies or the countries where they are located, but it indicated that the raids took place in 10 EU states.
German energy groups RWE and EON told AFP they were targeted by the inspections.
"We are cooperating with investigators," an RWE spokesman said. The company's German headquarters and its unit in the Czech Republic were visited.
While confirming the inspection, Martin Chalupsky, spokesman for RWE's Czech company, said the probe was related to a possible abuse of a dominant position in the European market by Russia's energy giant Gazprom.
Austrian firm OMV also confirmed it was visited by investigators and said it was "fully cooperating with the authorities".
Media reports said that Lithuania's Lietuvos Dujos, of which Gazprom owns a 37.1 percent share, was also targeted.
The European Union executive "suspects exclusionary behaviour, such as market partitioning, obstacles to network access, barriers to supply diversification, as well as possible exploitative behaviour, such as excessive pricing."
Officials said they are also probing "anti-competitive behaviour to the detriment of upstream suppliers themselves".
The opening of a probe does not necessarily mean fines or other penalties will follow, although the commission was quick to highlight similar investigations at national level that have previously opened up gas markets it deemed closed in Belgium, France, Germany and Italy.
Home to half a billion consumers in 27 states, the EU imports 60 percent of its gas requirements -- the majority of it from Russia.
In recent years, European consumers have faced a series of winter crises over these gas supplies, amid rows over pricing or political in-fighting across the former Soviet bloc.