WASHINGTON — The US Congress voted Wednesday to reduce sentencing disparities for offenders caught with crack cocaine versus the drug in powder form, a gap blamed for disproportionate jailing of black Americans.

The US House of Representatives, by voice vote, approved legislation the Senate passed in March, sending the measure to President Barack Obama to sign into law.

Critics of the US justice system's unequal treatment of the two drugs have argued that it disproportionately punishes African-Americans, who are convicted of crack possession in far greater numbers.

The bill cuts the 100:1 ratio in the amount of powder cocaine versus crack cocaine that trigger the same sentence, a disparity adopted in the mid-1980s amid fears a crack epidemic was fueling out-of-control violence.

The legislation as introduced by its lead author, Democratic Senator Dick Durbin, would have cut the ratio to 1:1, but dealmaking to ensure its passage resulted in a compromise ratio of 18:1.

The bill will reduce "the unwarranted lengthy sentences for crack cocaine that are devastating to African American communities," said Democratic Representative Jim Clyburn.

While just 25 percent of crack users are African-American, they constituted 81 percent of those convicted for crack offenses in 1987, according to the office of Senate Judiciary Committee chair Pat Leahy, a top backer of the bill.

That is "wrong and unfair, and it has needlessly swelled our prisons, wasting precious federal resources," Leahy said in a statement after the House action.

Under existing law, possession of five grams of crack cocaine -- roughly the weight of two sugar cubes -- triggers a mandatory minimum five-year prison sentence, the same triggered by trafficking 500 grams of powder cocaine.