A Rwandan genocide survivors' association on Wednesday condemned as "outrageous" a decision by an international tribunal to acquit on appeal two ex-army officers of charges related to the 1994 killings.

The UN-backed International Criminal Tribunal for Rwanda (ICTR) on Tuesday acquitted a Rwandan ex-paramilitary police chief and another former top officer of charges related to the genocide.

"It is a denial of justice," Jean-Pierre Dusingizemungu, the head of Ibuka, the genocide survivors' association, told AFP.

"These are men who planned the massacre of our people. Now the ICTR has outrageously allowed them to walk scot-free," Egide Nkuranga, the deputy head of Ibuka told the New Times.

General Augustin Ndindiliyimana was acquitted on the grounds that he did not have effective authority over the subordinates for whose abuses he had been sentenced.

Major Francois-Xavier Nzuwonemeye, the former commander of an elite battalion, was acquitted because the judges said his implication in the assassination of the prime minister and the killing of 10 Belgian UN blue helmets -- crimes in which men from his battalion took part -- had not been proven beyond reasonable doubt.

Dusingizemungu said the fact that the acquittal comes just as Rwanda prepares to commemorate the twentieth anniversary of the genocide is "a message liable to discourage" survivors.

The Arusha, Tanzania-based ICTR also reduced the sentence of a third officer, Innocent Sagahutu, a squadron commander in the elite reconnaissance unit, from 20 years behind bars to 15.

The ICTR said it would hand down its decision on a fourth officer, former army chief Augustin Bizimungu, at a later date.

READ: Rwanda ex-army chief jailed for 30 years over genocide

The UN-backed court was set up in late 1994 to try the alleged masterminds behind the genocide, in which an estimated 800,000 people, the overwhelming majority of them ethnic Tutsis, were killed.