Italy's flagship air carrier Alitalia has joined Air France-KLM and Delta Airlines in a trans-Atlantic joint venture that shares costs and revenue but stops short of a merger, the airlines announced Monday.

With Alitalia's addition, the joint venture controls one-quarter of all trans-Atlantic capacity with estimated annual revenues of $10 billion and will offer 250 flights and about 55,000 seats a day, the companies said in a statement. That includes 20 flights daily from Rome and Milan to five U.S. cities.

The expanded joint venture deepens cooperation on important trans-Atlantic routes among the four airlines, which are already part of the wider SkyTeam alliance.

"Trans-Atlantic traffic is the most strategic and competitive marketplace," Alitalia CEO Rocco Sabelli said in a statement.

Air France-KLM became the largest single shareholder in Alitalia with a 25-percent stake in January 2009, after its bid to buy the Italian carrier outright failed. The formerly state-run carrier is run by a group of Italian investors, who combined it with the much smaller rival Air One, after Alitalia shed its unprofitable assets in bankruptcy procedures.

The joint venture does not foresee an eventual Alitalia merger with Air France-KLM, Sabelli told a news conference in Rome attended by all four CEOs.

Sabelli said the deal "has the same benefits as a merger but maintains the companies separate as long as it makes sense. There is no plan for a merger."

The expansion of the joint venture to include Italy is meant to strengthen the partners' presence in the Italian market, fourth in Europe behind Britain, France and Germany.