WASHINGTON : US troops have reported more than 20,000 allegations of sexual assault at military installations over the past four years, the Pentagon said in a report.

The report, by the Defence Department's Sexual Assault Prevention and Response Office, details allegations by service branch for the fiscal years 2013-2016, with the Army - the largest branch of the military - reporting 8,294 cases in all.

The Navy meanwhile reported 4,788 cases, the Marine Corps 3,400 and the Air Force 8,876.

The military has about 1.3 million active-duty troops.

The publication of the report comes amid a sexual harassment scandal that has shaken America, with each new day bringing fresh allegations against powerful men from across the political, cultural and business landscape. While the Pentagon routinely tracks and releases data about assault within the ranks, Friday's study also specifies the bases on which the allegations were reported.

Since 2012, the Pentagon 's annual reports have reported drops in the numbers of sexual assault among the force, officials said.

In 2016, data showed a decrease from 6.1 percent of active duty women in 2012 to 4.3 percent in 2016, and from 1.2 percent of active duty men in 2012 to 0.6 percent in 2016.