Human Rights Watch has slammed Afghanistan's justice system after the country's top court upheld reduced sentences in the savage mob killing of a woman falsely accused of burning the Koran.

Farkhunda, 27, was beaten and her body set ablaze in broad daylight last March in Kabul, triggering angry nationwide protests and drawing global attention to the treatment of Afghan women.

An appeals court last summer slashed sentences of the men convicted of her murder, vacating the death penalty in four cases, reducing prison terms to 20 years in three others and 10 years in the fourth.

It also cut the sentences of nine other defendants.

The Supreme Court upheld those verdicts on Monday, sparking renewed criticism for what many call a blatant travesty of justice.

"It's a bitter irony that the latest blow to justice for Farkhunda occurred on the eve of International Women's Day," HRW said in a statement on Wednesday.

"At every stage of this case the Afghan criminal justice system failed to adequately investigate, hold to account or appropriately punish those responsible."

Farkhunda was attacked on the banks of the Kabul River after an amulet seller, whom she had reportedly castigated for peddling superstition, falsely accused her of burning a copy of the Koran.

Her case has become a symbol of the endemic violence that women face in Afghanistan, despite reforms since the hardline Taliban regime fell in 2001.

The backlash highlighted the angst of a post-Taliban generation in Afghanistan -- where nearly two-thirds of the population is under 25 -- that is often torn between conservatism and modernity as the country rebuilds after decades of war.