Washington (AFP) - Tens of thousands of U.S. federal prisoners convicted of small-time drug offenses will be able to seek sentence reductions under a new reform.

"This is a milestone in the effort to make more efficient use of our law enforcement resources and to ease the burden on our overcrowded prison system," said U.S. Attorney General Eric Holder, who has long encouraged the reduction of sentences for minor drug offenses, on Friday.

The U.S. Sentencing Commission voted to allow prison reductions for already-convicted and imprisoned inmates, in a move that is expected to affect some 46,000 convicts.

The move follows on the heels of the commission's approval of a reduction in sentences for future defendants several months prior.

"In the interest of fairness, it makes sense to apply changes to the sentencing guidelines retroactively," Holder said in a statement.

"At my direction, the Bureau of Prisons will begin notifying federal inmates of the opportunity to apply for a reduction in sentence immediately," he added.

A one-year implementation delay, however, will allow judges to assess who is eligible for the reductions.

Human Rights Watch welcomed the decision, which it said "will give 46,000 federal inmates serving unnecessarily long sentences for drug offenses a chance to seek sentence reductions."

HRW's U.S. advocacy director Antonio Ginatta hailed what she called "an important step away from the disproportionate punishment and racial disparities that have plagued federal drug sentencing for decades."

Nearly half of the 216,000 convicts now in federal prisons are doing time for drug-related crimes, in a country that spends $80 billion a year to lock up a staggering one-quarter of all the world's prisoners.

In August, Holder called the minimum jail terms under which they are sentenced "counterproductive" while noting the need to stay strict but be smarter about fighting crime.

And he warned that, while the total US population had increased by about a third since 1980, the prison population has soared by 800 percent.

In addition, he also aimed to bring sentences for possession of crack cocaine in line with those for powdered cocaine. Sentences for crack possession have been stiffer, and disproportionately meted out to African Americans.