White House spokesman Robert Gibbs said that walking away isn't a viable option to deal with a war that is about to enter its ninth year.

"I don't think we have the option to leave. That's quite clear," Gibbs said.

The debate over whether to send as many as 40,000 more U.S. troops to Afghanistan is a major element of a strategy overhaul that senior administration policy advisers will consider this week as they gather for top-level meetings on the evolving direction of the war.