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 Defense 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.