The Federal Aviation Administration said Thursday that it supported Chicago’s $15 billion master plan to expand O’Hare International Airport in an effort to reduce some of the United States’ worst flight delays.

In its environmental impact report, the agency said that it had considered a number of proposals, including ones that called for adding new runways, modifying the existing ones or leaving O’Hare alone and spending the money instead on building a third airport in the region.

But federal officials, who have been studying the matter since 2002, decided that the city’s solution was the best way to decrease airport congestion while doing the least amount of damage to the area’s water and air quality.

Calling O’Hare “the engine of the region’s economy,” Chicago Mayor Richard M. Daley on Thursday said that the airport expansion would begin after the FAA issues its final ruling on the plan. That is expected to come at the end of September, after a period of public comment.

The first new runway is set to open in 2007, and the entire project completed by 2013.