European regulators said Monday they had opened formal antitrust probes into members of two of the three global airline alliances.

The move follows months of informal talks between the European Commission and members of the Star and Oneworld alliances, amid intensifying scrutiny of industry tie-ups on both sides of the Atlantic.

The commission is unlikely to disband the alliances or fine members, according to people briefed on its plans, but may curb some of their existing and planned cooperation on routes, fares and marketing.

It said it suspects that some airlines under the broader Star and Oneworld banners may have cooperated too closely on pricing and capacity.

Commission spokesman Jonathan Todd said that it was investigating the matter as a matter of priority to "assess whether there is a violation of antitrust rules."

The Brussels-based commission targeted four Star members - Air Canada (AC.A.T), Continental Airlines Inc. (CAL), Deutsche Lufthansa AG (LHA.XE) and the United Airlines unit of UAL Corp. (UAUA).

The second investigation is into a proposed pact between members of the Oneworld Alliance, singling out AMR Corp.'s (AMR) American Airlines, British Airways Plc (BAY.LN) and Iberia Lineas Aereas de Espana SA (IBLA.MC).

The commission said the level of cooperation within the targeted alliances "appears far more extensive than the general cooperation between these airlines and other airlines which are part of the Star and Oneworld alliances."

One person briefed on its plans noted a formal probe was required before the commission could advance and seek any remedies, such as asking airlines to give up routes.

The third global alliance, SkyTeam, is already the subject of a formal Commission probe launched in 2006, with members including Delta Air Lines Inc. (DAL) and Air France-KLM (AF.FR) "testing" potential remedies to the regulator's concerns.

Star members recently won tentative approval from U.S. regulators to expand their cooperation to include the planned transfer of Continental from SkyTeam later this year.

BA, American and two other Oneworld members are seeking an antitrust exemption from U.S. regulators.

"This is a normal part of the E.U. process of examining our antitrust immunity application with American Airlines and Iberia," British Airways said in a statement.

Lufthansa said the company is in "constructive talks" with the commission, and aims to implement the broader Star pact as quickly as possible.

The other U.S. airlines involved in the probes said they were reviewing the commission statement.

Airline regulators have already fined airlines more than $1.5 billion for operating cartels related to fuel surcharges levied by passenger and cargo airlines.

-By Peppi Kiviniemi and Doug Cameron, Dow Jones Newswires; +32 (0)2 741 1483; peppi.kiviniemi@dowjones.com