UPDATE 2-Exxon CEO doubts Obama's alternative energy goal

(Adds breakdown of U.S. renewable energy supplies, comment on a carbon tax)

By Tom Doggett

WASHINGTON, Jan 8 (Reuters) - Exxon Mobil (XOM.N: Quote, Profile, Research) CEO Rex Tillerson said on Thursday it would be difficult to meet President-elect Barack Obama's goal to significantly boost U.S. alternative energy production.

In a speech at George Mason University in Fairfax, Virginia, Obama said he wanted the United States to double its output of alternative energy sources during the next three years as part of his plan to revive the American economy.

"I think that's going to be very challenging to do," Tillerson told reporters following a speech at the Woodrow Wilson Center in Washington.

He said the United States will not be able to double biofuels output during that period and there was not enough manufacturing capacity to build the wind turbines needed to meet Obama's goal.

"I'm not wanting to be critical of his aspirational goal. Aspirational goals are good because they challenge us all to go out and find the right answer," Tillerson said of Obama's plan.

But he added: "Let's be realistic about timeframes. Let's not fool ourselves."

Despite calls for more alternative energy use, he said, "for the foreseeable future" the world will rely on oil and natural gas for 60 percent of its energy needs.