The United Nations says attackers from South Sudan's second-largest ethnic group have forced their way into a UN peacekeeping mission base in conflict-wracked Jonglei state, pursuing civilians who had taken refuge there.
UN deputy spokesman Farhan Haq said on Thursday the UN fears "there may have been some fatalities but can't confirm who and how many at this stage."
He said the UN had lost contact with the Akobo base and at least three peacekeepers and one civilian worker were unaccounted for.
Rapidly escalating ethnic violence has raised fears of instability in the world's newest country.
Haq said the situation in the capital, Juba, appeared to have calmed to some extent, but he noted unconfirmed reports of several students killed by security personal at Juba University on Wednesday.
He said thousands have sought refuge and called for UN protection.
Anti-government fighters in the country have claimed control over the flashpoint town of Bor as rival military factions paid no heed to the president's call for talks, raising fears of a slide into civil war.
On Thursday, President Salva Kiir held talks in Juba with seven east African foreign ministers seeking to end the conflict, the first international initiative since clashes erupted this week.
Battles between troops loyal to Riek Machar, South Sudan's fugitive former vice president, and the government's military continued early on Thursday.
"Our soldiers have lost control of Bor to the force of Riek Machar late on Wednesday," army spokesman Philip Aguer told AFP news agency.
"There was shooting last night. We don't have information on casualties or the displaced in the town, as operations are ongoing."
The fighting started on Monday shortly before Kiir announced that security forces had put down an attempted coup by supporters of his former deputy.
At a press conference on Wednesday Kiir said he was willing to sit down with the former vice president for talks, but said "I do not know what the results of the talks will be."
Talking to Al Jazeera's Hannah McNeish, Machar denied that any plot was carried out to coup Kiir
"My life was in danger; my colleagues were being arrested for no reason. They are not plotters, it was not a coup. Nobody wants that," Machar said, claiming he was "used as a scapegoat" by Kiir to purge the ruling SPLM party of rivals to avoid reforming it.
About 20,000 people have sought refuge at UN facilities in Juba, since fighting started on Sunday, and on Tuesday the United States ordered its citizens to leave South Sudan immediately.