More than 1,000 South Sudanese are languishing in jail in Sudan, official said.
The Refugee Council of Sudan Coordinator, Ms Acchol Malong Deng, confirmed that the inmates were held in different parts of Sudan.
Some, she disclosed, were awaiting their death sentences, without disclosing the charges they faced.
“The situation of the South Sudanese prisoners in Sudan is really bad and is getting worse,” Ms Deng was quoted saying by the Eye Radio in Juba.
She added that Khartoum often executed those sentenced to death without informing their next of kin.
“They always execute and take the dead bodies to the morgue, only for the relatives to be surprised by the news of the death on paying a visit to prison,” she said.
Attempts to get a comment from the Sudan embassy in Juba were futile.
In 2011, Juba accused Khartoum of holding more than 50,000 South Sudanese in different detention centres in slave-like conditions.