The U.S. Defense Department said on Thursday the estimated number of sexual assaults in the military climbed nearly 38 percent in 2018 compared with a survey two years earlier, data that critics say laid bare broken Pentagon promises of a crackdown.

The Pentagon said there were 6,053 reports of sexual assaults last year, according to an anonymous, bi-annual survey. It is the highest since the U.S. military began collecting this kind of survey data in 2004.

Moreover, actual sexual assaults were likely to be about triple the reported number, given the estimate that just one out of three victims in the military file a complaint. The military survey estimated 20,500 male and female service members experienced some kind of sexual assault last year. The estimated number in 2016 was 14,900.

"It is clear that sexual assault and sexual harassment are persistent challenges," Acting Defense Secretary Patrick Shanahan said in a statement.

"To put it bluntly, we are not performing to the standards and expectations we have for ourselves or for each other. This is unacceptable."

Based on a survey taken only every two years, the report said that not only the number but the prevalence of sexual assault was on the rise in 2018.

Around 6.2% of women in the Department of Defense experienced sexual assault or unwanted sexual contact in 2018, compared to 4.3% two years earlier, according to the report.

The level for men was much lower and relatively stable: 0.7% experienced assault in 2018, slightly higher than 0.6% two years ago.

The problem was worst in the Marines: some 10.7% of women in that service reported sexual assault last year, compared to 7.5% in the Navy and lower rates in the other services.

Sexual assaults on men from all services was in the 0.7-0.8% range.

But the overall estimates for assaults reported and not reported underscore the problem for both genders: last year, the Pentagon estimates, more than 13,000 women were assaulted and more than 7,000 men.

The report showed that the youngest women were at the highest risk, and that their attackers were like them most often of low rank, equal to or slightly higher than the victim.

That pointed to the need for more understanding of how younger recruits interact, especially with the rise of social media, according to experts.

"We have to look and see what is ... happening with the 17-24 year-olds that our programs are not accounting for," said Nate Galbreath, deputy director of the Pentagon's Sexual Assault Prevention and Response Office.

"That is one the things that we want to investigate: to kind of see what risks are posed by using social media sites."

Shanahan told a Congressional panel Wednesday that said he supported a proposal to seek a specific crime for sexual harassment under the military's unique justice system.

"We will criminalize certain activities in this next year to reflect the seriousness that we take on certain behavior," he said.

Senator Kirsten Gillibrand, a Democratic presidential candidate who has been an advocate for overhauling rules for prosecution of sex crimes in the U.S. military, said the report made clear that it was time for Congress to act.

"Sexual assaults continue to increase dramatically while the number of cases going to trial goes down," she said. "The status quo is not working."