WASHINGTON - The number of reported sexual assaults in the U.S. armed forces jumped by 50 percent last year, with almost three-quarters of the cases prompting military prosecutions.

Defense Secretary Chuck Hagel said the figures, contained in a 2013 annual report on sexual assault he released Thursday, show that victims have more faith in the system thanks to protections he's implemented.

"Sexual assault is a clear threat to the lives and the well-being of the women and men who serve our country in uniform," Hagel told reporters at the White House. "It destroys the bonds of trust and confidence that lie at the heart of our armed forces."

The Pentagon has been grappling with the problem of military sexual assault for decades.

Hagel has made what on Wednesday he called "this despicable crime" a top priority since becoming Pentagon chief in February 2013. The report showed 5,061 claims of military sexual assault last year, up from 3,374 in 2012. A decade ago, by contrast, there were fewer than 1,700 such claims.

But Nate Galbreath, senior executive adviser in the Pentagon's Office of Sexual Assault Prevention and Response, said the spike in cases does not actually reflect more instances of assault, just increased reporting of such alleged crimes.

Protect Our Defenders, an advocacy group based in Burlingame, Calif., cast doubt on the Pentagon's assertion that the number of assaults last year did not actually increase.

"Until the release of DOD's biannual survey later this year, we will not know the reason for this increase [in reported assaults] - which could be due to more troops being attacked or a result of the intense public attention to this issue," said Nancy Parrish, the group's president.

She said the Pentagon "has released no evidence to prove its claim that victims have more trust in the existing military justice system or the treatment they have received."

In a surprising statement, Hagel cited "estimates that men comprise more than half the victims of sexual assault in the military." That claim is at odds with a 2012 Pentagon anonymous survey in which 6 percent of women in uniform said they had experienced some form of "unwanted sexual contact," compared with 1.2 percent of men in uniform.