The US Senate bowed to Pentagon pressure on Thursday by refusing to remove the authority to prosecute rapes and other serious crimes from US military commanders.

The vote was 55-45 in favour of stripping commanders of that authority, but that fell short of the 60 necessary to move ahead on legislation sponsored by Senator Kirsten Gillibrand.

It would have given the decision to take serious crimes to courts-martial to military trial lawyers independent of the chain of command.

The debate and vote were the culmination of a nearly year-long campaign to curb sexual assault in the ranks.

But Pentagon leaders opposed the measure, as did some former prosecutors and military veterans in the Senate.

However backers of the measure insisted that previous piecemeal reforms only had a limited impact on a problem.

Recent survey results have suggested that some 26,000 people in the ranks, mostly women, have been sexually assaulted, with thousands unwilling to come forward for fear of inaction or retribution.

Including Senator Gillibrand, the Bill had the support of 17 of the Senate's 20 women.

Sen Gillibrand is expected to pursue further measures in the spring, when the Senate's armed services committee begins work on a defence policy Bill for 2015.

In two hours of debate supporters of the Bill argued on the Senate floor based on their personal experiences, showing frustration with what they dismissed as fixes around the edges and horrific stories from the ranks.

"The current system is failing the men and women in uniform," said Democrat Senator John Walsh, who is the first Iraq war veteran in the body.

"We have moved too slowly."

After blocking Sen Gillibrand's Bill, the Senate overwhelmingly passed a weaker measure to eliminate the "good soldier defence," which allows a service member's character and military performance to be used in a defence.