CHICAGO - A new airport or vast expansion of one of Chicago's existing airports will be necessary to keep pace with booming demand for air travel in the coming decades, the head of the Federal Aviation Administration said Wednesday.

That's in addition to an ongoing $15 billion expansion of O'Hare International Airport pushed through by Chicago Mayor Richard Daley, Robert Sturgell, the FAA's acting administrator, told The Associated Press.

"Mayor Daley has done a great job in transforming Chicago with a plan, but they need another airport as well," said Sturgell, who didn't immediately offer details on where a new airport could be built in the nation's third largest city.

Chicago plays too vital a role as an aviation hub not to further upgrade airport capacity, Sturgell said, adding that he appreciated the fierce resistance such projects can generate, including from residents worried about noise and air pollution.

"It takes a lot of local political will to move new runways and airports forward," he said in a telephone interview from Washington, D.C.

Other cities like New York and Atlanta will also need new airports or ambitious expansion projects, he said.

More than a dozen new runways have opened in the United States since 2000, with three more opening this November, including at O'Hare.

But Sturgell said that won't suffice over the long run.

Despite a drop in flights this year with high fuel prices cutting into airline profits, the number of air travelers could double to 1 billion annually over the next decade, Sturgell said. And since newer, streamlined planes will carry fewer passengers, the total number of planes flying in and out of airports could increase at an even faster rate, he said.

In a 2007 report, the FAA noted that just two major airports have opened in the last 40 years - DallasFort Worth and Denver International. As many as four would have to be built over the next 20 to 30 years, he said.

New airports and runway expansions would have to happen in conjunction with implementation of a new satellite-guided air traffic system, dubbed NextGen, Sturgell said. That would replace the current, radar-based system, which deploys 1950s-era radar technology.

The newer, more efficient system is expected to cost more than $30 billion, and the FAA wants to have it fully operational by the 2020s. But questions about just how to install it and about who should pay what costs have raised doubts about whether it will be up and running by then.