European Union antitrust authorities Tuesday raided the offices of several pharmaceutical firms, as the bloc intensifies its scrutiny of efforts by brand-name drugmakers to prevent generic competitors from bringing their products to market.

The European Commission, the European Union's executive arm, announced the raids but didn't name any of the companies involved. French drugmaker Sanofi-Aventis (SAN.FR) and Teva Pharmaceuticals Industries Ltd. (TEVA), an Israeli generic drug maker, said their offices were raided.

A Teva spokeswoman said the inquiry focused on the French generic drug market.

"We believe that Teva France has conducted its business in a responsible manner," she said.

While the commission declined to comment on the suspected violations, it has been investigating the deals that brand-name companies strike with their generic competitors to delay bringing generic medicines to market. E.U. antitrust commissioner Neelie Kroes in July raised concerns that these deals indicate the two segments of the drug industry have agreed not to compete with each other.

"There is something rotten" with the deals, she said. The commission is aware of at least 200 settlement agreements between brand-name and generic drugmakers, many to delay the entry of generic medicines to the market, Kroes said.

The U.S. Justice Department in July said it would focus special attention on such deals.

Generic medicines have become a major threat to brand-name drugmakers, which have struggled in recent years to develop new drugs as their older products lose patent protections.

The inspections announced Tuesday are among the first steps in a commission antitrust probe, which can often take years to complete.

Merck KGaA (MRK), Bayer AG (BAYN.XE), GlaxoSmithKline PLC (GSK), UCB SA (UCB.BT) and Solvay SA (SOLB.BT) weren't among the firms raided, spokespeople for the companies said Tuesday.

-By Matthew Dalton, Dow Jones Newswires; +32 2 741 1487; matthew.dalton@dowjones.com

(Marietta Cauchi, Thomas Gryta, Peppi Kiviniemi and Mimosa Spencer contributed to this article.)