The United States raised alarm Saturday over repeated claims that both South Sudan's government and rebels are breaking a deal to stop weeks of bloodshed in which thousands have been killed.
Ceasefire monitors were deployed this week amid clashes that have continued despite the deal signed last month by the government and rebels, brokered by the East African bloc known as the Intergovernmental Authority on Development (IGAD).
"We are deeply concerned by reports of violations by both the government of South Sudan and anti-government forces of the cessation of hostilities agreement that was signed in Addis Ababa on January 23," US State Department spokeswoman Jen Psaki said in a statement.
"We urge the redeployment or phased withdrawal of foreign forces invited by either side, and warn of the serious consequences which could result from any regionalization of this conflict."
She called on the South Sudanese government to facilitate the ceasefire monitors' "important work" to allow both sides to report any breaches of the agreement.
Psaki also urged Juba to support an IGAD-led political dialogue and reconciliation process, and to release four remaining political detainees.
"The expeditious release and transfer of all of the detainees would reduce tension and build confidence in an inclusive reconciliation process," she said.
IGAD teams are meant to ensure the warring sides honor their deal, but many fear the unarmed observers will struggle to monitor loose frontlines between multiple forces in a vast country with few roads.
Fighting erupted in South Sudan in mid-December between two factions of the army -- troops loyal to president Salva Kiir and those who supported his former deputy Riek Machar, himself a seasoned guerrilla fighter.
It quickly spread throughout the country as the rebels, under the nominal command of Machar, joined forces with ethnic militia.
The ceasefire agreement failed to put an end to the fighting, in which thousands have died and almost 900,000 been forced to flee their homes.