Image copyright Getty Images Image caption The Justice Department orders federal prosecutors to seek maximum punishments for criminals

The US attorney general has ordered federal prosecutors to seek harsher criminal sentencing, undoing an Obama-era policy to ease prison overcrowding.

In a two-page memo, Jeff Sessions instructed US attorneys to "charge and pursue the most serious, readily provable offence".

He has vowed to crack down on violence and drugs at the Justice Department.

The move is a reversal of ex-President Barack Obama's policy to reduce jail time for low-level drug crimes.

"It means we are going to meet our responsibility to enforce the law with judgment and fairness," Mr Sessions said on Friday. "It is simply the right and moral thing to do."

The new criminal justice policy "ensures that the Department enforces the law fairly and consistently, advances public safety and promotes respect for our legal system", he said in a memo released to the public on Friday.

Mr Sessions' predecessor, Eric Holder, had instructed prosecutors in 2013 to avoid pursuing the maximum punishment for criminals in cases such as minor drug offences, which would have triggered mandatory minimum sentencing.

Mandatory minimum sentences laws, which were passed in the 1980s and 1990s as part of the US "war on drugs", prevent judges from applying discretion when sentencing certain drug offences and are instead determined by the quantity of drugs involved in the crime.

Media playback is unsupported on your device Media caption Why does Wisconsin send so many black people to jail?

Is this a new era? Jessica Lussenhop, BBC News, Washington

When President Trump first appointed Jeff Sessions to become the next attorney general, prison reform advocates feared that there would be a return to the old, tough-on-crime, war-on-drugs policies of the '80s and '90s. This directive is the beginning of the fulfilment of that prophecy.

Mandatory minimum sentences for non-violent drug crimes had fallen out of favour in the Obama-era and were criticised for being overly harsh with no proven positive impact on public safety. Efforts to undo draconian sentencing practices were backed by legislators on both sides of the aisle, one of the few issues both parties agreed on.

Barack Obama presided over the first decline in the federal prison population since the 1970s and used his power of executive clemency to free or reduce the 1,927 federal inmates, many for drugs charges.

In going back to old Bush-era sentencing policies, Sessions is essentially rejecting any notion that these practices disproportionately affect people of colour, crowded prisons and caused the modern era of mass incarceration, and are a drain on taxpayer resources. Reform advocates say Sessions is rekindling the failed "War on Drugs" - and Sessions himself points to the opioid epidemic as the reason he wants to reinstate aggressive sentencing.

Despite bipartisan support, Congress has failed to pass sentencing reform bills in the past, and it will be interesting to see if this move by the Trump Department of Justice reinvigorates that effort or completely stamps it out.

Mr Obama had sought to ease mandatory minimum sentences to reduce jail time for low-level drug crimes and help relieve overcrowded prisons in the US as part of criminal justice reform.

The 2013 policy also encouraged prosecutors to omit details about drug quantities in cases of non-violent offenders with no previous charges or ties to gangs or cartels to avoid harsher punishments.

Mr Sessions' guidance rescinds Mr Holder's approach, directing prosecutors to disclose all information about a case to the courts and follow current sentencing rules.

Prosecutors are also required to get approval first before seeking a different sentence.

Image copyright Getty Images Image caption Mr Obama (above) aimed to reduce prison populations and ease punishment for lower drug offenders

Mr Sessions has argued an uptick in violence in US cities and the opioid epidemic underscores a need for tougher law enforcement.

"In 2015, more than 52,000 Americans died from a drug overdose," he said on Friday. "We intend to reverse this trend."

The dramatic shift is seen as part of President Donald Trump's tough campaign rhetoric to crack down on criminals and boost law enforcement.