The human rights group, Amnesty International, has launched a campaign urging Belarus and Uzbekistan to stop using the death penalty.
Some families have not been given relatives' bodies for burial (picture Amnesty International)
It says they are the last former Soviet republics to still use the punishment.
Amnesty said people in both countries were sentenced to death in unfair trials, often after "confessions" extracted through torture.
It added that prisoners were often not told the date of execution and burial places remained secret.
Neither Belarus nor Uzbekistan has released full statistics on the number of people they execute by shooting.
In 2001, the Uzbek authorities said that about 100 people were executed annually, a figure contested by Uzbek human rights groups who say that the real number is double that.
In Belarus, Amnesty says the number of people sentenced to death is thought to have decreased to between four and seven each year.
Amnesty is concerned that the secrecy surrounding the death penalty, as well as the conditions on death row, lead to immense suffering.
It says prisoners are frequently beaten by prison officials and held in small cells with only limited and monitored contacts with the outside world.
"Honestly, they treat us here not like human beings but as if we were cattle or small mosquitoes," said Uzbek prisoner Zhasur Madrakhimov in a letter he managed to smuggle out before being executed in 2004, eight days after the UN Human Rights Committee had urged the authorities of Uzbekistan to stay his execution.
The impact on prisoners' families is also a concern for Amnesty.
Tamara Chikunova, whose son Dmitry was executed in 2000 in Uzbekistan, said: "It is one of the worst things for me that I do not know where Dmitry is buried.
"If I knew I would at least have a place where I can go with my grief and where I can talk to him."
She has erected a symbolic gravestone for her son in a cemetery in Tashkent, next to the grave of his grandfather.
After the collapse of the Soviet Union in 1991, all the newly independent states retained the death penalty.
Since then, nine have abolished it - Armenia, Azerbaijan, Estonia, Georgia, Latvia, Lithuania, Moldova, Turkmenistan and Ukraine - and four have suspended it - Kazakhstan, Kyrgyzstan, Russian Federation and Tajikistan.