US Attorney-General Jeff Sessions is directing federal prosecutors to pursue the most serious charges possible against the vast majority of suspects.

The move is a reversal of Obama-era policies that is sure to send more people to prison and for much longer terms.

The move has long been expected from Sessions, a former federal prosecutor who cut his teeth during the height of the crack cocaine epidemic and who has promised to make combating violence and drugs the Justice Department's top priority.

"This policy affirms our responsibility to enforce the law, is moral and just, and produces consistency," Sessions wrote in a memo to US attorneys.

Advocates quickly criticised the move as a revival of the worst aspects of the drug war, which they say subjected non-violent, lower-level offenders to unfairly harsh sentences that disproportionately hurt minority communities.

"It looks like we're going to fill the prisons back up after finally getting the federal prison population down," said Kevin Ring, president of Families Against Mandatory Minimums.

"But the social and human costs will be much higher."

The announcement is an unmistakable undoing of Obama administration criminal justice policies that aimed to ease overcrowding in federal prisons and contributed to a national rethinking of how drug criminals were prosecuted and sentenced.

Sessions contends a spike in violence in some big cities and the nation's opioid epidemic show the need for a return to tougher tactics.

He foreshadowed the plan early in his tenure, when he signalled his strong support for the federal government's continued use of private prisons, reversing another Obama directive to phase out their use.

"We know that drugs and crime go hand-in-hand," Sessions said in a Friday speech.

"Drug trafficking is an inherently violent business. If you want to collect a drug debt, you can't file a lawsuit in court. You collect it by the barrel of a gun."

The policy memo says prosecutors should "charge and pursue the most serious, readily provable offence" - something more likely to trigger mandatory minimum sentences.

Those rules limit a judge's discretion and are typically dictated, for example, by the quantity of drugs involved in a crime.