Rebel fighters gather in a village in Upper Nile State February 8, 2014. U.S. State Dept. expresses concern about reported violations of a cease-fire between the government and rebels. (Goran Tomasevic/REUTERS)

The United States expressed concern Saturday about reported violations of a shaky cease-fire between South Sudan’s government and rebel forces and urged the removal of foreign fighters engaged in the conflict.

Thousands of people have been killed and more than half a million have fled their homes since fighting erupted in December. The conflict has brought the oil-producing country, the world’s newest, to the brink of civil war.

State Department spokeswoman Jen Psaki said the United States is “deeply concerned by reports of violations by both the government of South Sudan and anti-government forces” of the cease-fire deal.

“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,” Psaki said in a statement.

President Salva Kiir’s government and rebels loyal to his fired deputy, Riek Machar, agreed to the truce Jan. 23.

Regional and world powers worry about the potential for a spillover of violence in an already volatile part of Africa. South Sudan declared independence from Sudan in 2011.

The rebels have accused government forces and fighters from the Sudanese Justice and Equality Movement — a rebel group from north of the border — of razing Machar’s home town, massacring women and children as they fled. The government also accuses the rebels of breaking the cease-fire.

An advance team sent by East African nations to monitor the truce arrived in South Sudan last Sunday. Violence erupted in the capital, Juba, in December before spreading across the country.

Psaki said the United States welcomed the arrival in Juba of the first component of the monitoring team, adding that “we strongly urge the government of South Sudan to facilitate [the team’s] important work, which will provide both sides with a mechanism to report any breaches of the agreement.”

She reiterated a U.S. request for the government to release the remaining four of a group of 11 detainees held on suspicion of trying to stage a coup.

“The expeditious release and transfer of all of the detainees would reduce tension and build confidence in an inclusive reconciliation process,” Psaki said.

— Reuters