The decision on whether to prosecute serious crimes such as sexual assaults in the US military has shifted from commanders to independent lawyers, the Pentagon said Thursday, implementing a congressionally-mandated reform.

The change followed years of pressure from victims' advocates to ensure better accountability in the military. Congress passed a law requiring the shift in 2021 and US President Joe Biden ordered its implementation in July.

"Prosecutorial discretion for 13 serious criminal offenses will be shifted away from commanders to specially trained and independent judge advocates who reside within the Offices of Special Trial Counsel and report directly to the secretaries of the military departments," Defense Secretary Lloyd Austin said in a statement.

"This landmark change to the US military justice system will significantly strengthen the independent prosecution of sexual assault and other serious criminal offenses in the Department of Defense," he said.

Removing the investigation and prosecution of sexual assault cases from the chain of command was one of the main proposals made by an independent commission set up by Austin to look into the handling of sexual assault in the military.

Previously, commanders were responsible for making decisions about pursuing serious crimes allegedly committed by their subordinates — a system critics said often presented conflicts of interest and other issues.

A senior US defense official said ahead of the change that it marks a "monumental improvement of the military justice system," and that victims can be assured their cases "will be handled professionally and consistently with the best practices and procedures of civilian prosecution offices."

According to the latest Pentagon data, there were 8,942 reports of sexual assault involving military personnel in 2022, slightly up on the previous year.