A UN peacekeeping base in South Sudan's Jonglei State has been attacked and there are reports that some people have been killed.
Deputy UN Secretary-General Jan Eliasson said: "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."
UN spokesman Farhan Haq said the situation in Jonglei has deteriorated.
He said Lou Nuer youth have reportedly forced entry into the UN Mission in South Sudan (UNMISS) Temporary Operating Base to reach civilians who went there for protection.
Mr Haq said the mission will try to "extract unarmed UN personnel from Akobo while reinforcing the base in Akobo with additional 60 troops from Malakal tomorrow".
South Sudanese government troops battled to regain control of a flashpoint town and sent forces to quell fighting in a vital oil producing area, in the fifth day of a conflict that has deepened ethnic divisions in the two-year-old nation.
The conflict, which has so far killed as many as 500 people according to local reports received by the United Nations, has alarmed South Sudan's neighbours.
African mediators held talks with President Salva Kiir on Thursday to try to broker peace.
Mr Haq said the situation in central Juba appeared to have calmed down somewhat, allowing limited movement of UN personnel, though the United Nations continues to receive reports of civilians seeking protection.
"Following unconfirmed reports of several students killed by security personnel in Juba University yesterday, several hundred students reportedly remaining on campus have requested assistance from the UN Mission in South Sudan," he said.
"In another location in Juba called the Kator complex, approximately 2,000 to 5,000 civilians have sought refuge and have called for UNMISS force protection from the UN Mission. A patrol is en route."