US Attorney General Eric Holder has called for major changes to the country's criminal justice system, announcing measures aimed at easing prison overcrowding.

The measures will scale back the use of harsh prison sentences for certain drug-related crimes, divert people convicted of low-level offences to drug treatment and community service programmes and expand a prison programme to allow for release of some elderly, non-violent offenders.

In a speech at the American Bar Association in San Francisco on Monday, Holder said he is mandating a change to Justice Department policy so that low-level, non-violent drug offenders with no ties to large-scale organisations, gangs or cartels will not be charged with offences that impose mandatory minimum sentences.

Mandatory minimum prison sentences - a product of the government's war on drugs in the 1980s - limit the discretion of judges to impose shorter prison terms.

Under the altered policy, the attorney general said defendants will instead be charged with offences for which accompanying sentences "are better suited to their individual conduct, rather than excessive prison terms more appropriate for violent criminals or drug kingpins".

'Vicious cycle'

Federal prisons are operating at nearly 40 percent above capacity and hold more than 219,000 inmates - with almost half of them serving time for drug-related crimes and many of them with substance use disorders. In addition, 9 million to 10 million prisoners go through local jails each year.

We need to ensure that incarceration is used to punish, deter and rehabilitate - not merely to convict, warehouse and forget. Eric Holder, Attorney general

Holder praised state and local law enforcement officials for already instituting some of the types of changes Holder says must be made at the federal level.

Aggressive enforcement of federal criminal laws is necessary, but "we cannot simply prosecute or incarcerate our way to becoming a safer nation," Holder said.

"Today, a vicious cycle of poverty, criminality and incarceration traps too many Americans and weakens too many communities. However, many aspects of our criminal justice system may actually exacerbate this problem, rather than alleviate it," he said.

"We need to ensure that incarceration is used to punish, deter and rehabilitate - not merely to convict, warehouse and forget."

The attorney general said 17 states have directed money away from prison construction and toward programs and services such as treatment and supervision that are designed to reduce the problem of repeat offenders.

In Kentucky, legislation has reserved prison beds for the most serious offenders and refocused resources on community supervision. The state, Holder said, is projected to reduce its prison population by more than 3,000 over the next 10 years, saving more than $400m.

He also cited investments in drug treatment in Texas for non-violent offenders and changes to parole policies which he said brought about a reduction in the prison population of more than 5,000 inmates last year. He said similar efforts helped Arkansas reduce its prison population by more than 1,400.