Eyder Peralta returned to his base in Kenya yesterday but his South Sudanese assistant is still being held by authorities, Isabel Lara told The Associated Press in an email.
"We are in touch with authorities regarding his release," Lara said.
South Sudan officials did not respond to requests for comment.
Peralta's detention was the latest in a crackdown on foreign journalists in South Sudan, the East African nation that gained independence in 2011 but has experienced renewed civil war.
At least 13 foreign journalists have been denied media accreditation or visas to South Sudan so far this year. An AP reporter was detained and expelled from the country in December.
More From This Section
Since then, the situation for journalists has gotten worse, and some have been detained for publishing articles critical of the government, according to the UN report.
According to the Committee to Protect Journalists, at least five journalists have been killed in South Sudan since it gained independence in 2011.
South Sudan ranks 145th in a press freedom survey released last week by Reporters Without Borders.