Gates' comments on the subject was in response to a letter from Senator Hillary Clinton requesting briefings on contingency planning for a future withdrawal of US forces.

In his response, Gates rebuffed the request but promised to find a way "to keep you apprised of the conceptual thinking, factors, considerations, questions and objectives associated with draw down planning."

"Further, you may rest assured that such planning is indeed taking place with my active involvement as well as that of senior military and civilian officials, and our commanders in the field," Gates said.

"I consider this contingency planning to be a priority for this Department," he wrote.

The letter offered no details on the planning or possible scenarios for a reduction in US forces.

General David Petraeus, the US commander in Iraq, and US Ambassador Ryan Crocker are supposed to report to Congress in mid-September on the impact of a surge in US forces and what should be done next.