WASHINGTON -- U.S. senators dressed down senior military leaders Tuesday, led by female lawmakers, combat veterans and former prosecutors who insisted that sexual assault in the ranks has cost the services the trust and respect of the American people as well as the nation's men and women in uniform.

Summoned to Coingress, Army Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, and the beribboned four-star chiefs of the service branches conceded in an extraordinary hearing that they had faltered in dealing with sexual assault. One said assaults were "like a cancer" in the military.

But they strongly opposed congressional efforts to strip commanders of their traditional authority to decide whether to level charges in their units.

Members of the Senate Armed Services Committee, especially the panel's seven female senators, grilled the chiefs about whether the military's mostly male leadership understands differences between relatively minor sexual offences and serious crimes that deserve swift and decisive justice.

"Not every single commander necessarily wants women in the force. Not every single commander believes what a sexual assault is. Not every single commander can distinguish between a slap on the ass and a rape because they merge all of these crimes together," said Sen. Kirsten Gillibrand, a Democrat.

Frustration among the senators seemed to boil over as they discussed recent high-profile cases and statistics on sexual assault that underscored the challenges the Defence Department and Congress face.

Sen. John McCain, a Republican and a Navy veteran of Vietnam, said a woman came to him the previous night and said her daughter wanted to join the military. She asked McCain if he could give her his unqualified support.

"I could not," McCain said. "I cannot overstate my disgust and disappointment over the continued reports of sexual misconduct in our military. We've been talking about the issue for years, and talk is insufficient."

The committee is considering seven legislative proposals, including one introduced by Gillibrand that would deny commanders the authority to decide when criminal charges are filed and remove the ability of senior officers to convene courts-martial.

Dempsey and the service chiefs warned against making such dramatic changes. Removing commanders from the military justice process, Dempsey said, would undercut their ability to preserve good order and discipline in their units.

"We cannot simply legislate our way out of this problem," said Gen. Ray Odierno, the Army's chief of staff. "Without equivocation, I believe maintaining the central role of commander in our military justice system is absolutely critical to any solution."

But Gillibrand defended her proposal, which has garnered 18 co-sponsors in two weeks. She said victims of sexual assault are reluctant to report crimes to their commanders because they fear their allegations will be dismissed and they might face retaliation. Aggressive reforms in the military's legal code are needed to force cultural changes, she said.

"You have lost the trust of the men and women who rely on you," Gillibrand said. "They're afraid to report. They think their careers will be over. They fear retaliation. They fear being blamed. That is our biggest challenge right there."

Dempsey and the service chiefs told the committee they back Defence Secretary Chuck Hagel's April recommendation to change the Uniform Code of Military Justice and largely strip commanding officers of the power to toss out a military verdict. That change is included in several of the Senate proposals including Gillibrand's and is likely to be adopted by the House Armed Services Committee on Wednesday in its version of the annual defence policy bill.

The Pentagon estimated in a recent report that as many as 26,000 military members may have been sexually assaulted last year, up from an estimated 19,000 assaults in 2011, based on an anonymous survey of military personnel. While the number of sexual assaults that members of the military actually reported rose 6 per cent to 3,374 in 2012, thousands of victims were still unwilling to come forward despite new oversight and assistance programs aimed at curbing the crimes, the report said.