Friends of the imprisoned international peace activist Peter Biar Ajak, who was seized and held without charges in his homeland of South Sudan, say he now has been charged with treason-related offenses by the government.

Ajak, 34, has been outspoken in his criticism of the country's repressive regime.

Now a well-known scholar and speaker on East Africa, he had come to Philadelphia as a teenage refugee in January 2001, one of 40,000 "Lost Boys" left homeless by the violence and brutality of civil war in Sudan. He graduated from Central High School and La Salle University, earned a master's degree at Harvard, and was completing a doctorate at Cambridge University in England at the time of his arrest.

Among the charges are concealing treason, publishing or communicating false statements prejudicial to South Sudan, and an offense that covers insurgency, banditry, sabotage or terrorism, according to a charging document.

The maximum sentence for concealing treason is 20 years in prison. That offense is defined as knowing that someone inside or outside the country is trying or planning to overthrow the government, and then failing to share that information with authorities. the document states.

>>READ MORE: International rights activist jailed in South Sudan got his start in Philadelphia 

His friends shared the charging document that lists allegations against Ajak, who was seized by national security police on July 28 as he boarded a plane in Juba, the capital of South Sudan.

The Facebook group Free Peter Biar said on Thursday that Ajak had been charged and would be going to court. There was no immediate confirmation from the South Sudan government. A spokesperson for Amnesty International, which earlier issued an Urgent Action call for Ajak's release, said it was checking into news of the charges.

Nyagoah Pur, a South Sudan specialist for Human Rights Watch, said the agency has seen a list of charges that has begun to circulate on social media, but has no independent confirmation from Ajak's family or a legal representative.

"We believe his continued detention is linked with the peaceful exercise of his freedom of expression. Any charges will most likely be trumped up," Pur said, adding that the government has used arrests, detention and false charges as a means to intimidate and silence critics.

A spokesperson for U.N. Secretary-General Antonio Gutteres said that the organization's mission in South Sudan was monitoring the situation, and that the mission's human-rights team was in contact with government authorities.

This month, Sens. Cory Booker (D., N.J.) and Chris Coons (D., Del.), both members of the Foreign Relations Committee, said they were deeply concerned by Ajak's detention and by the forced disappearances of both Dong Samuel Luak, a South Sudanese human-rights lawyer and activist, and Aggrey Idri, a government critic and opposition member.

Both were reportedly detained without charges by South Sudan's National Security Service, and government authorities have failed to acknowledge their detention, bring charges or release them, the senators said.

Ajak's detention at a notorious South Sudan prison, known as the Blue House because of its tinted windows, has led to worldwide calls for his freedom. Supporters have launched social-media campaigns under the #FreePeterBiar hashtag and @freepeterbiar on Twitter.

Political prisoners held at the Blue House can face starvation or death, and Amnesty International has documented cases of detainees being tortured with knives and beaten with bamboo sticks.

Ajak has called for a new, peaceful generation of young leaders to rise and run South Sudan. He has criticized how President Salva Kiir and former deputy turned rebel leader Riek Machar have conducted peace talks, which resulted in the signing of a power-sharing agreement this month. A similar deal fell apart in 2016.