The European Union and the United States agreed Thursday to expand a three-year-old accord that allows airlines to operate more freely across the Atlantic. The move will increase access to each other's markets and narrow differences over environmental regulations, but industry executives were disappointed that no agreement was reached to remove the remaining barriers to foreign ownership and control of airlines. Siim Kallas, the European Union transport commissioner, hailed the agreement, saying it would remove 'constraints on foreign investment and market access that have been holding back the development of the most important aviation market.' Kallas said he would seek approval by the bloc's 27 transport ministers and the European Parliament by the end of June. The preliminary deal reached Thursday builds on a ground-breaking March 2007 'open skies' pact that since 2008 has allowed any European or American airline to fly between any two airports in either region -- a move that has significantly increased competition in the $18 billion trans-Atlantic market. It comes after years of negotiations between European officials and their counterparts from the Transportation Department. Under the terms of the agreement reached Thursday, the United States said it would seek to ease restrictions that currently limit foreign ownership of voting stock in American carriers to 25 percent. But the issue still faces significant opposition in Congress and from labor groups that are concerned about the effect of such a change for an important sector of the U.S. economy. Washington also agreed to give European carriers new commercial opportunities to fly between the United States and third countries in the future, once existing European restrictions on airport noise pollution were lifted. John R. Byerly, U.S. deputy assistant secretary for transportation affairs, acknowledged there was no timetable for changing U.S. legislation on foreign ownership, and Daniel Calleja, the chief negotiator for the European Commission, said there was no timetable for changing the noise laws in Europe. The European Union has predicted that the provisions in the agreement could increase annual trans-Atlantic traffic by as much as 26 million passengers, to about 75 million by 2013, and generate up to 12 billion euros in economic benefits, like lower fares for consumers. 