The United Nations says a group of South Sudanese youth has forced its way into a U.N. peacekeeping mission base in conflict-wracked Jonglei state, pursuing civilians who had taken refuge there.

U.N. deputy spokesman Farhan Haq said Thursday the U.N. fears "there may have been some fatalities but can't confirm who and how many at this stage."

"Our base in Akobo, Jonglei State, was attacked and we have reports that lives are lost. We don't have the details of that yet," Deputy U.N. Secretary-General Jan Eliasson told reporters, according to Reuters.

"The U.N. Mission in South Sudan (UNMISS) will try to extract unarmed U.N. personnel from Akobo while reinforcing the base in Akobo with additional 60 troops from Malakal tomorrow," Haq said.

Rapidly escalating ethnic violence has raised fears of instability in the world's newest country.

Haq says the situation in the capital, Juba, appears to have calmed to some extent, but he noted unconfirmed reports of several students killed by security personal at Juba University on Wednesday.

He says thousands have sought refuge and called for U.N. protection.

South Sudanese government troops battled to regain control of a flashpoint town and sent forces to quell fighting in a vital oil producing area Thursday, Reuters reported.

The conflict, which has so far killed as many as 500 people, has alarmed South Sudan's neighbors. African mediators held talks with President Salva Kiir on Thursday to try to broker peace, Reuters reported. 

The Associated Press contributed to this report.

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