The European Commission said in a statement on Friday that such deals could breach EU antitrust rules.

"The goal is to ensure that this tie-up does not harm passengers on EU-U.S. routes," the Commission said as it announced the start of the investigation.

It said it would consider the implications of the joint venture compared to the situation where the airlines would otherwise be competing.

The agreements signed between the airlines in 2009 and 2010 cover coordinated transatlantic operations in terms of capacity, schedules, pricing and revenues.

"The parties also share profits and losses of their transatlantic flights," said the Commission, which acts as competition regulator for the European Union.

Alliances allow their members to streamline costs, share revenues and increase scale, a better option for airlines than mergers, which can be difficult and expensive to achieve.

The Commission also said it was closing separate formal antitrust proceedings against several members of the SkyTeam airlines alliance: AeroMexico (AEROMEX.MX), Air France and KLM (AIRF.PA), Alitalia, Czech Airlines, Delta, Korean Air Lines (003490.KS) and Continental Airlines, which is part of United Continental (UAL.N).

(Reporting by Foo Yun Chee and Rex Merrifield; Editing by Jon Loades-Carter)

• A Permanent Economic Emergency