The United States' ambassador to South Sudan on Thursday called on the government to release a prominent activist and economist who has been held without charge for five days.
Peter Biar Ajak, who has featured on international media outlets to analyse the ongoing peace talks in Khartoum, was arrested Saturday at Juba International Airport, according to his family and fellow activists.
"We are asking for his release and for more information about really what the case has been about," US Ambassador Thomas Hushek told journalists shortly after meeting Justice Minister Paulino Wanawilla to discuss the situation.
Biar's arrest has provoked condemnation from rights groups, including the South Sudan Young Leaders Forum, which he founded.
"We are deeply troubled that no one has been allowed to speak or visit him at National Security headquarters in Jebel," said a statement from the forum.
Philip Sanyang Ngong, a rights activist who works with Advocates Without Borders, has confirmed that Biar is being held at the notorious National Security Service compound known as "Blue House".
However the justice minister claimed not to know anything about the arrest.
"We are still waiting of course for information from the arresting authority," Wanwilla told reporters.
"We don't have information still but obviously any arrest may have a reason behind it and until we find out what is the reason we will not be able to talk about a case which we have not yet taken note of."
Biar was one of the so-called Lost Boys who were displaced or orphaned during the Sudanese Civil War, before South Sudan became independent in 2011.
Biar went to the United States in 2001 where he studied Economics in Philadelphia.
He later got his Masters in Public Administration from Harvard and was currently a PHD student at Cambridge, according to an online profile of him by the International Growth Council thinktank, where he worked as a researcher.
He worked as a World Bank economist based in South Sudan and advised the government on economic policy.
In recent weeks he has urged both President Salva Kiir and his rival Riek Machar to step aside and make way for new people to rule the country, ravaged by nearly five years of war.
South Sudan's warring parties last week signed a preliminary power-sharing deal which sees Machar re-instated as vice president, with four other vice president positions shared out among other political groups.