The on Tuesday urged to release a freelance journalist with the state-owned broadcaster Voice of America detained during weekend protests.
Diing Magot was arrested along with five demonstrators who were staging a protest on Sunday over the soaring prices of basic goods in the impoverished country, police said.
South Sudan's police spokesman Major General Daniel Justine told journalists an investigation into Magot's case was ongoing.
The US embassy in Juba issued a statement seeking the immediate release of the South Sudanese reporter.
"We affirm the right of journalists to do their jobs without interference or harm," it said on Twitter.
That call was echoed by the head of the Union of Journalists of South Sudan.
"It is clear she has not committed any offence and the law is not being followed," said Oyet Patrick, adding that Magot should have already been taken to court if there was a case against her.
Journalists have been targeted by the security services in recent months.
In June, several were briefly detained in parliament, accused of illegally covering a press conference by the opposition-allied deputy speaker.
"Freedom of the press is extremely precarious in South Sudan, where journalists work under constant threat and intimidation, and where censorship is ever-present," said media rights watchdog Reporters Without Borders.
South Sudan, like many countries in sub-Saharan Africa, has suffered from the fallout of Russia's war on Ukraine.
The conflict has sent global prices of food and fuel soaring.
The value of the local currency has also slumped over the past month, with 700 pounds buying one US dollar compared with 450 in early July.
South Sudan, one of the poorest countries on the planet despite large oil reserves, has suffered from civil war, natural disasters, hunger, ethnic violence and political infighting since it gained independence in 2011.
The World Food Programme warned in March that more than 70 percent of South Sudan's 11 million people would face extreme hunger this year because of natural disasters and violence.
This article is part of °®Âþµº's States of Journalism series, a sustained exploration of freedom, repression, and accountability in MENA and global media landscapes. Read more of the series' articles .