WASHINGTON - The number of sexual assaults reported in the U.S. military rose 11 percent last year, the Defense Department said Tuesday, but Pentagon officials conceded that they still don't know how common sexual assaults are because many troops fear retribution if the attacks come to the attention of their commanders. Despite the suspected under- reporting, sexual assault is more common in the military than it is among the civilian population, the report suggests - two for every 1,000 service members, versus 1.8 per 1,000 civilian women and one per 1,000 civilian men, according to statistics compiled by the Family Violence Prevention Fund. During the past year, the Defense Department has campaigned to encourage victims of sexual assault to come forward, and officials said they think that effort led to the increase. "Our goal was to get more people to report" assaults, said Kaye Whitley, the director of the Defense Department's sexual assault prevention and response office, which compiled the report. Women in the military, in particular, are reluctant to come forward for fear of appearing weak or being ostracized for reporting a fellow soldier, Pentagon officials say. Many women also complain that they've been accused of being gay under the military's "don't ask, don't tell" policy after rebuffing unwanted advances from their male colleagues. Since 2005, the military has tried to overcome that reluctance by creating two categories of reporting. So-called "unrestricted" cases are reported to the victim's superiors, and an investigation is launched. In "restricted" cases, the victim's commanders aren't notified and there's no further investigation, but the victim can receive medical and mental-health care. Both categories rose in 2009, the Pentagon's report showed - with restricted reports numbering 837 in 2009, compared with 753 in 2008. Unrestricted reports rose to 2,516 from 2,265. Of the unrestricted reports, 2,061 involved men assaulting women, up more than 10 percent from 1,864 in 2008. The number of men reporting assaults by other men also rose, to 173 in 2009 from 123 in 2008, a 40 percent increase. There was a single report of a woman being assaulted by a woman; there was none in 2008.