As the Pentagon prepares for a major buildup of American troops in Afghanistan, US Defence Secretary Robert Gates has warned that his country cannot become bogged down in the unrealistic goal of turning the devastated country into an economically prosperous nation.

Instead, Gates said the US must limit its focus to ensuring terrorists do not regain control of the region and use it to coordinate attacks.

"If we set ourselves the objective of creating some sort of Central Asian Valhalla over there, we will lose because nobody in the world has that kind of time, patience or money to be honest."

President Barack Obama has vowed to shift military resources away from Iraq and move them toward Afghanistan and Pakistan, which he says is the central front in the struggle against terrorism and extremism.

In a plan initiated during the Bush administration and endorsed by Obama, the Pentagon is planning to double the 34-thousand contingent of US forces in Afghanistan.