WASHINGTON — The number of US service members who reported being sexually assaulted surged by 50 percent last year, the Pentagon announced Thursday, the latest sign of how the military has struggled to cope with sex crimes in the ranks.
Military officials said they didn’t know whether the sharp increase meant that more crimes were committed, but they added that evidence suggested more victims were willing to come forward. The Pentagon has been conducting a high-profile campaign to prevent sexual assault and punish offenders amid concerns that it neglected the problem for years.
At a minimum, however, the startling figures released Thursday illustrated how sexual violence is much more prevalent in the armed forces than commanders realized.
The Marine Corps, for instance, recorded an 86 percent increase in sexual assault reports during the fiscal year that ended Sept. 30, 2013. The Army saw a 51 percent jump during the same period, compared with 46 percent for the Navy and 33 percent for the Air Force.
All told, the Pentagon said it received 5,061 reports of sexual assault last year, up from 3,374 the year before. Officials said they presume that represents only a fraction of the number of sex crimes committed each year against the 1.4 million active-duty members of the military. As in society at large, they said, most victims are reluctant to report what happened to them.
‘‘We have a long way to go in solving this problem,’’ Defense Secretary Chuck Hagel told reporters. ‘‘We must keep up the pressure and intensify our efforts to improve victim confidence in our system.’’
The Pentagon had reported a preliminary version of the numbers in December. The final statistics released Thursday mirrored those results, but the military also provided a detailed breakdown that offered more insights:
About 79 percent of sexually assaulted service members were women (women constitute about 15 percent of the 1.4 million troops on active duty). At the same time, Pentagon officials said they were concerned that the number of victimized men was especially under-reported and that male troops are more reluctant to report being abused.
A slight majority — 54 percent — of the reports filed last year involved a member of the military who had been attacked by someone else in uniform. The remainder involved a mix of civilian and military perpetrators and victims.
Ten percent of those filing reports said they had been victimized before joining the military but waited to come forward — in some cases, more than a year later.
The Defense Department defines sexual assault as ‘‘intentional sexual contact characterized by the use of force, threats, intimidation, or abuse of authority’’ when the victim does not consent. It can cover a variety of crimes.
Slightly more than half the cases investigated last year were for the reported offenses of rape, sexual assault, or non-consensual sodomy, according to the report.
Advocacy groups credited the Pentagon for taking steps to support victims and encourage them to come forward. But some said they still had reservations about whether the military is properly investigating and prosecuting sex offenses once they are reported.
‘‘There is definitely a seriousness about it,’’ Nancy Duff Campbell, co-president of the National Women’s Law Center in Washington, said about the military’s overall responsiveness to sexual assault in the ranks. ‘‘I’m still quite concerned about the whole justice system. There’s still a lot of problems with how the cases are handled.’’
Pentagon figures show that 2,149 service members were investigated last year on suspicion of committing sexual assault. Of those, about 73 percent received some form of punishment, ranging from court martial to administrative discipline.
The number of suspected offenders who were convicted at court martial, however, remained relatively small: about 11 percent of all criminal investigations into sexual assault.
Even fewer — about 6 percent — were convicted of a crime serious enough to require the defendant to be added to a sex-offender registry.
Defense officials said those figures reflect the inherent difficulty of bringing sexual assault cases to trial and winning convictions. Physical evidence is often lacking and it can be hard to establish whether consent was not given.
Nate Galbreath, a Pentagon official and the lead author of the report released Thursday, said that most sexual assault victims in the military were enlisted troops under age 25. Their attackers were usually senior in rank.
‘‘Our offenders tend to be a little bit older, and a little bit higher in rank,’’ he said. ‘‘These are not strangers. These are people that they know, and that’s the tragedy.’’
Alcohol was a factor in as many as half the cases, officials said. In response, Hagel said he had ordered the military to conduct a broad review of alcohol policies. Defense officials said they weren’t contemplating alcohol bans, but wanted to find ways to discourage the kind of binge drinking that often precedes sexual assaults.