Colonel Polly Kenny, the Staff Judge Advocate for the Second Air Force and the leader of the prosecution team, said no physical contact between trainers and recruits is ever allowed.

"There is no time when an instructor should touch a trainee," Kenny told reporters before the start of today's proceedings.

"That power imbalance has to be in place to conduct the training mission, in order to turn the sons and daughters who come to us into airmen of character."

Kenny said all 10 of Walker's alleged victims are expected to testify, and she said the prosecution's case should take five to six days. Lead defense attorney Joseph Esparza has not said what Walker's defense will be, or whether Walker will testify.

Walker is the first Lackland drill sergeant to stand trial in connection with the case, which has grown into the worst military sex scandal since one at Aberdeen, Maryland, U.S. Army base in 1996.

The scandal has prompted an examination of whether men and women should continue to be trained together and there have been calls for congressional hearings. Nearly one in five Air Force recruits is a woman, the highest of all the military branches.

Walker is being tried by a seven member panel of officers and enlisted personnel. Under military court rules, two-thirds of the panel is needed to convict Walker, unlike in a civilian jury trial, where the verdict must be unanimous.

(Editing by Greg McCune and Philip Barbara)