The U.S. military saw a sharp increase in the number of service members who say they have experienced a sexual assault despite its efforts to curtail the issue, according to a report released by the Department of Defense on Thursday.

The report, based on the results of an anonymous survey, estimated that 20,500 service members — 13,000 women and 7,500 men — across the Army, Navy, Marines, Air Force and the National Guard experienced “contact or penetrative sexual assault” while serving in the military in 2018. This is a noticeable uptick from the 14,900 service members who reported similar incidents the last time the military conducted the anonymous survey in 2016, and it is a significantly higher number than those who filed a formal report. Just 6,053 service members made a formal report in fiscal year 2018, the Pentagon noted.

The issue particularly affects women serving across each branch of the military, according to the report, but women from 17-24 years old were found to be particularly at risk. Incidents involving active duty men remained “statistically unchanged” compared to 2016, the report noted.

Roughly 6.2 percent of active duty women reported experiencing a sexual assault via the anonymous survey, which is up from the 4.3 percent of women who reported in 2016. The results of the survey also found that women in the Marine Corps reported experiencing the highest rate of sexual assaults at roughly 11 percent, up from 7 percent in 2016, followed by the Navy, Army and Air Force, respectively. Those branches also saw an uptick in estimated rates of assault, according to the survey results.

Roughly one in five women who experienced sexual harassment while serving also experienced sexual assault, even when controlling for pay, service and deployment status, the report said.

The results of the survey also indicated that the vast majority of sexual assaults of service members occurred between people who “work, train, or live in close proximity,” according to the report, and that “military women indicated that offenders were most often military men whom they considered to be a friend or acquaintance, acting alone.”

Acting Defense Secretary Patrick Shanahan said in a letter to senior department officials on Thursday that the Pentagon will implement additional policies to curb and try to eliminate sexual assault in the military.

"To put it bluntly, we are not performing to the standards and expectations we have for ourselves or for each other. This is unacceptable," Shanahan said in the letter. "We cannot shrink from facing the challenge head on. We must, and will, do better."

Dr. Elizabeth Van Winkle, the executive director of Department of Defense office of force resiliency, said she was “disheartened and frustrated” at the numbers during a press briefing Thursday, but said that the department is committed to addressing the issue.

“They undercut the order and discipline necessary to maintain the strongest military force in the world…the order and discipline our Service members deserve,” she said. “This is men and women seeing things and doing things that contradict our core values, choosing not to take action, potentially because they don’t know how, they aren’t empowered to, or they don’t know they need to. This is a Department problem, and we are all responsible for fixing it.”

She added, “Sexual assault is counter to our expectations for our force...but I am not without hope.”

Despite the increase in reported incidents of sexual assault, the report does highlight some silver linings.

For instance, the department said in the report that it believes policies put in place over the past decade have encouraged greater reporting of sexual harassment and assault. In 2006, one out of 14 service members who experience sexual assault reported it to a superior. In 2018, one in three service members reported incidents, the report said.

The department also established a sexual assault accountability task force at the request of Sen. Martha McSally, R-Ariz., in March. McSally, a retired Air Force colonel and the first female fighter pilot to fly a combat mission, revealed at an Armed Services Committee subcommittee hearing on military sexual assault in March that she was raped by a superior officer while on active duty.

Shanahan said in his letter to DoD officials on Thursday that the department also plans to make sexual harassment a "stand alone" crime in the military justice system.