Sen. Kirsten Gillibrand (D-N.Y.) said on Monday that a large number of sexual assault cases in the U.S. Military continue to "remain in the shadows." In this photo, Gillibrand speaks at the Center for American Progress 10th Anniversary policy forum in Washington on Oct. 24, 2013. Reuters/Yuri Gripas

Just days after the Pentagon said its initiatives to tackle sexual assault in the military were paying off, a U.S. senator has accused the Defense Department of hiding the actual extent of such crimes. Kirsten Gillibrand, D-N.Y., said on Monday that a large number of cases continue to "remain in the shadows," according to a report by the Associated Press.

"I don't think the military is being honest about the problem," Gillibrand reportedly said, after analyzing over 100 sexual assault cases. Spouses of service members and female civilians living near military facilities are most likely to be assaulted, she said, adding that these cases are not counted by the Pentagon while determining the prevalence of sexual assault in the military.

Moreover, of the 107 cases she reviewed, less than a quarter went to trial and just 11 resulted in conviction for sexual assault, Gillibrand told the AP. In several cases, the victims do not testify because they have been presumably intimidated, she reportedly said.

"It's frustrating because you look at the facts in these cases and you see witnesses willing to come forward, getting the medical exam and either eventually withdrawing their case or the investigators deciding that her testimony wasn't valid or believable," Gillibrand reportedly said.

Gillibrand's comments come just four days after a Defense Department report indicated that about 20,300 military personnel had admitted they were victims of "unwanted sexual contact" in 2014, compared to about 26,000 in 2012. The report also found an increase in the number of people willing to report such incidents. In 2014, the military received 6,131 reports of sexual assault involving service members as either victims or subjects, an increase of 11 percent over 2013.

However, despite the department's latest revelations, Gillibrand, who had reportedly sought details of sexual assault cases investigated between 2009 and 2014 at four U.S. military bases, said the Pentagon's unwillingness to provide the requested data called into question the "department's commitment to transparency and getting to the root of the problem."