The European Union Commission said Monday it has launched two antitrust cases looking into airline cooperation on transatlantic routes that could constitute an illegal cartel.

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

The first investigation concerns both existing and planned cooperation between members of the Star Alliance group - Air Canada (AC.A), Continental Airlines (CAL), Deutsche Lufthansa (LHA.XE) and United Airlines (UAUA).

The second investigation is into proposed deals between three members of the Oneworld Alliance - American Airlines (AMR), British Airways (BAY.LN) and Iberia (IBLA.MC).

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

The agreements that prompted the European Commission's scrutiny allow the airlines to coordinate their commercial, marketing and operational activities on routes between the European Union and North America.

The commission said it is concerned the airlines' plans to jointly manage schedules, capacity, pricing and revenue on transatlantic routes may lead to reduced competition on the routes.

In the U.S., the alliances have recently come under scrutiny from the House of Representatives transportation committee, which is proposing a bill that would unwind the existing system of global alliances.

Committee Chairman Rep. James Oberstar has said the alliances have become too dominant in the marketplace, with too much control over passenger fares.

In the U.S., airline alliances can apply for antitrust immunity that protects them from regulators. Star Alliance currently enjoys such immunity for some of the partners to jointly set prices and schedules across the Atlantic.

But the probe is likely to prove tricky for Continental Airlines, which is in the process of switching from rival alliance SkyTeam to Star and needs renewed antitrust immunity to work more closely with United.

Oneworld Alliance, the smallest of the three groupings that dominate transatlantic flights, is also trying to win immunity in the U.S.

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

However, the commission's antitrust spokesman Jonathan Todd said it was misleading to call the investigation a routine inspection.

We are investigating this as a matter of priority to "assess whether there is a violation of antitrust rules," Todd added.

If found guilty, the commission can fine the companies up to 10% of their global revenue and demand they make changes to their business practices.

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