(Reuters) - An Air Force drill sergeant accused of serial sexual assault repeatedly sent texts and photos to one victim, including a photo of him shirtless, wearing boxers and with his military hat on, a woman identified as victim No. 6 testified at a military court martial on Monday.

Accused Staff Sergeant Luis Walker, wearing his blue Air Force dress uniform, spoke only briefly as first court martial of a widening military sex scandal got underway at Lackland Air Force base in San Antonio, Texas.

Most of the day was spent with lawyers sparring over the rules of evidence. Victim No. 6 testified as part of one motion to determine if the photo of a shirtless Walker could be entered in evidence.

"He sent a lot of photos," she testified, glancing at Walker. "I remember them clearly."

The photos were sent to her after she finished basic training but before she went to technical school at Lackland. According to Air Force rules, trainers are forbidden from private communication with recruits during or after formal training.

Walker is facing a total of 28 counts, including rape, sexual assault, and attempted sexual assault of a total of 10 female recruits and faces life in prison if convicted.

His case opened a floodgate of allegations of misconduct between trainees and Military Training Instructors at Lackland, which is the home of all Air Force enlisted basic training.

Five other instructors have been charged with crimes, another six are under investigation, and 31 women have come forward to say they were victims of improper sexual contact by their drill sergeant. Another 35 instructors and one squadron commander have been removed from their posts.

VARIETY OF CHARGES

There are about 500 instructors at Lackland.

Walker is charged with having sexual relations with four of the 10 trainees, who are only identified by number. He is also charged with "flirting" with female trainees, sending them sexually explicit text messages, making inappropriate comments like telling one woman to "get naked," and forcing one trainee to touch his genitals.

He allegedly threatened some of the women, telling them that he would damage their military careers if they reported on his conduct.

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.