A leading international rights group has slammed Afghanistan's justice system over the case of a woman beaten to death by a frenzied mob last year.

Human Rights Watch says the system failed Farkhunda Malikzada when it cut the sentences of the men convicted of her murder.

The New York-based group also says it's a "bitter irony" that Afghanistan's Supreme Court confirmed the reduced sentences on March 8, the International Women's Day.

Four men had been sentenced to death for their role in the killing of Malikzada, falsely accused of burning a Quran at a Kabul shrine.

The Supreme Court upheld sentence reductions to 20 years for three of the men and 10 years for the fourth. Additionally, nine other sentences handed down in the case were also reduced.