PhD student Peter Biar Ajak, 35, is being "arbitrarily detained in a modern-day hellhole", his lawyer says.
Mr Ajak has been detained without charge since his arrest at Juba Airport in July last year. He had returned to his home country to hold a youth forum, but was intercepted by government officials and taken into custody.
'A clear violation of his rights under international law'
Jared Genser, representing Mr Ajak, claims this was "in clear violation of his rights under international law".
The charges being considered by the South Sudanese authorities include terrorism and treason, according to Mr Genser, both of which carry the death penalty.
Shortly before his return to the country, Mr Ajak, who has a wife and two young children, made the following comment on Twitter about the country's governance: "We must stop thinking that the so-called leaders will bring peace #SouthSudan. We, the great people of #SouthSudan, must organize ourselves to bring about the peace we deserve!"
As a child he was one of the country's Lost Boy refugees, orphaned and displaced by war. Mr Ajak resettled in the United States and studied at Harvard University and Philadelphia's La Salle University, before moving to the UK to continue his studies at Cambridge University, according to a BBC report.
Cambridge University "deeply concerned"
Amnesty International, the human rights organisation, is campaigning to raise awareness of his plight, which was raised in the United States House of Representatives by Congresswoman Madeleine Dean, who "knew Peter as a brilliant student and leader" while teaching at La Salle University.
Seif Magango, Amnesty International's deputy regional director for East Africa, described Mr Ajak's detention as "absurd".
"South Sudanese authorities must either release him so he can rejoin his wife and children who miss him dearly, or charge him with an offence recognised under international law," he said
A spokesperson for Cambridge University said: "The university remains deeply concerned about Peter's welfare and his access to legal representation and the violation of his rights in accordance with the constitution of South Sudan, which guarantees all South Sudanese people liberty and security of person, due process, and freedom of expression and association."
There has been no comment from the South Sudan government.
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