The number of sexual assaults in the US military increased by 13 percent in fiscal year 2021, reaching a record high, according to an annual report published Thursday by the Pentagon.

The Department of Defense's Sexual Assault Prevention and Response Office (SAPR) said there were 8,866 reported assaults "involving service members as either victims or subjects" in the year to September 30, 2021, compared to 7,813 the previous year.

But only a portion of sexual assaults are reported to authorities, and the SAPR, using surveys of the troops, estimates that nearly 36,000 active duty servicewomen and men -- 8.4 percent of women and 1.5 percent of men -- experienced unwanted sexual contact during the year.

The Pentagon said it could not "scientifically determine if there was a true increase," due to a change in the metric used to measure sexual assault.

But other data also pointed to an increase, "suggesting an overall growth in unhealthy military climate" since 2018, the report said.

"Our numbers indicate that this is the highest sexual assault estimated prevalence rate for women" since the issue was first studied closely in 2006, said Elizabeth Foster, executive director of the Pentagon's Office of Force Resiliency, which focuses on the well-being of service members.

For men, the level of assaults is the second highest; the highest level was in 2006.

"These numbers are tragic, and extremely disappointing," Foster said.

The sharpest rise in assaults came in the army, up by 26 percent, followed by the navy, up 19 percent, and two percent each in the air force and marines.

In January, President Joe Biden issued an order making sexual harassment a crime under military law.

The order means sexual assaults, domestic violence and assaults on minors will now be tried before a military court, and that the decisions on taking cases to court will be entrusted to specialized prosecutors, rather than officers in the military chain of command.

Senior officers have been accused of habitually ignoring, covering up, or treating lightly sexual assault claims in the past.

The military had resisted changes, insisting the previous system had better served the need of maintaining discipline in the ranks.

But because previous efforts to get the problem under control had failed, Defense Secretary Lloyd Austin appointed an independent commission to submit recommendations as to how best to deal with perpetrators of sexual violence in the military.

The commission concluded that removing the power to prosecute or not prosecute cases from the command hierarchy was the only way to tackle it.