South Sudan's army said on Saturday they had "lost communication" with commanders battling rebels in a key oil state where rebels have seized a major town, amid a worsening conflict engulfing the young nation.
However, army spokesman Malaak Ayuen said the situation was "all calm" in the flashpoint town of Bor, where at least 58 people were killed and more than 100 others wounded when gunmen stormed a UN base on Thursday.
The UN Security Council called the attack on the camp where thousands of civilians were sheltering an "outrage" that may "constitute a war crime".
Among the dead were children, and the UN has warned that the death toll could increase.
The top UN official in the war-torn nation, Toby Lanzer, praised peacekeepers for preventing what could have been a massacre of up to 5,000 people, and vowed the world body would use "lethal force" again to protect civilians under their protection.
In the north of the country in the troubled oil-state of Unity, the army is fighting rebels loyal to sacked vice president Riek Machar, after the insurgents launched a renewed offensive targeting the young nation's key oil fields.
Rebel fighters recaptured the town of Bentiu on Tuesday, with UN peacekeepers reporting corpses littering the streets.
Bentiu, one of the most bitterly contested regions in the war, is the first major settlement to have been retaken since they started a renewed assault.
Rebels said the army had fled Bentiu in disarray and had seized large amounts of military equipment including artillery, but the army dismissed the reports and said it was staging a counterattack.
However, military headquarters in the capital Juba said they had not been able to get through to troops fighting on the ground since Thursday.
"There is a problem of lost communication...the telephones are not going through," Ayuen said.
It was not immediatly clear what has happened to the government force in Unity state, but rebels are reportedly extending attacks, including in the Melut and Renk areas of Upper Nile, another oil-producing state.
The conflict in South Sudan, which only won independence from Sudan in 2011 and is the world's youngest nation, has left thousands dead and forced around a million people to flee their homes.
The fighting has been marked by reports and allegations of atrocities by both sides, with ethnic massacres, child soldier recruitment and patients raped and murdered in hospitals by attacking forces.
The surge in fighting in the four-month-long conflict comes amid warnings by UN Secretary General Ban Ki-moon that more than one million people are at risk of famine in the war-torn country.
The fighting is between soldiers loyal to President Salva Kiir against mutinous troops who sided with Machar, sacked as vice-president in 2013.
The conflict has also taken on an ethnic dimension, pitting Kiir's Dinka tribe against militia forces from Machar's Nuer people.
Lanzer said measures had been taken to boost security at other UN bases in the country, which are sheltering close to 60,000 people from different ethnic groups.
"This past week has been the most bleak in South Sudan's history," Lanzer said on Friday.