"She will be sorely missed," Rumsfeld said in a statement announcing her departure. He praised her as a "gifted communicator."

"During her remarkable two years of service in the Department of Defense, she has developed countless new methods to tell the story of our fighting forces, and bring their courage, dedication and professionalism into sharp focus for all Americans," he said.

Clarke's biggest innovation was to arrange for intensive on-the-spot coverage of the war in Iraq by embedding hundreds of reporters from US and international news organization with a US-led invasion force.

The experiment produced a crackling, sometimes chaotic journalistic record of a military campaign in real-time and on an unprecedented scale.

Rumsfeld later said it was a gamble but agreed to do it after being persuaded that the virtues of American soldiers in combat would shine through the coverage.

Clarke said she was leaving for personal reasons but did not elaborate.

"I depart sadly, because this has been the best professional experience of my life. It has been a true honor working for the men and women of the US military."

She was the first woman to serve as assistant secretary of defense for public affairs and the chief spokesman for the US military.

Unlike most of her predecessors, she had no previous experience with the military and worked only briefly as a journalist.

But she drew on long experience in politics, government and public relations in helping Rumsfeld sell Americans on the need for military action, as well as a radical restructuring of the military, to deal with new threats like terrorism and weapons of mass destruction.

She had served as press secretary for former president George Bush's unsuccessful re-election bid in 1992, as a spokeswoman for the US Trade Representative's Office, and as an aide to Senator John McCain.

She was head of the Washington office of Hill and Knowlton, a leading lobbying and public relations firm when Rumsfeld hired her as his press secretary.

Rumsfeld aide Lawrence Di Rita will perform her duties until a permanent replacement is confirmed, the Pentagon said.

WAR.WIRE