NEW YORK   -   The US military has reported a major spike in sexual assaults despite years of efforts to address the problem.

Figures show 20,500 instances of unwanted sexual contact occurred in 2018, up from 14,900 in 2016 which is the last time a survey was conducted. Alcohol was involved in one third of cases, and most victims were female recruits ages 17 to 24.

Acting Defense Secretary Patrick Shanahan has told Congress he plans to “criminalise” military sex assault. The report released on Thursday surveyed the Army, Navy, Air Force and Marines, and found 20,500 cases in 2018.

Incidents of unwanted sexual contact - which ranges from groping to rape - rose by around 38% between 2016 and 2018. Only one out of three cases were reported to authorities, the report found.

In more than 85% of cases, victims knew their attacker. The majority of cases involved young women whose attacker was often a superior officer. The report should be “a trip wire”, said Nate Galbreath, Deputy Director of the Department’ of Defense’s Sexual Assault Prevention and Response Office.

“This is what tells us that there’s something going on that we need to hone in on,” he told ABC News. “We’ve got a higher prevalence for women 17 to 24. We’re going to be focusing very, very tightly on that.”

Congresswoman Jackie Speier, who chairs the House Armed Services Committee’s personnel panel, told the USA Today newspaper that the military “must accept that current programmes are simply not working”.

“Congress must lead the way in forcing the department to take more aggressive approaches to fighting this scourge,” she said, calling for intervention from US lawmakers.

On Wednesday, Mr Shanahan revealed some of the recommendations made to him by the Sexual Assault Accountability and Investigation Task Force, which was created last month after the urging of Senator Martha McSally.

Senator McSally, who was the first female US fighter pilot to fly in combat, revealed in March that she had been raped by a superior officer while serving in the Air Force. Mr Shanahan said during a congressional hearing that he plans to “criminialise” sexual assault.

It is unclear if he would need congressional approval to make changes to the Uniform Code of Military Justice - the US military’s legal code, to make the offense a stand-alone crime.