McClellan also said there are no plans for any immediate announcements after journalists asked if the president was due to unveil a new initiative backing space travel to Mars.
"I think as long as this review is underway, it would be premature to get into any speculation about our space policy that has been and continues to be under review," McClellan said.
Asked if Bush plans to make an announcement about US aims in space on the 100th commemoration of the first powered flight in North Carolina on December 17, 1903, by the Wright brothers, McClellan replied: "I don't announce events this far in advance."
Bush will map out a future policy for the US presence in space once an interagency review process has made its recommendations to the White House, McClellan explained.
The review process was set up in the wake of the February 1 tragedy in which the NASA shuttle Colombia broke up over the western United States while returning to earth, killing all seven astronauts onboard.
Two NASA rovers are due to land on Mars in January in order to evaluate possible signs of life on the Red Planet.