UN peacekeeping mission UNMISS said on Wednesday that they had twice been prevented from accessing Pajok.
Juba: More than 6,000 people have fled fighting in southern South Sudan, with refugees recounting the slaughter of civilians by armed forces, the United Nations refugee agency said on Friday.
Fighting between government forces and rebels erupted on Monday in the town of Pajok in the country's southern Equatoria region, a previously peaceful part of the country that has seen a surge in conflict in recent months.
According to a statement from the UNHCR over 6,000 people have fled into the northern Ugandan district of Lamwo since the attack while many others were hiding in the bush trying to find their way to safety in Uganda as main roads were blocked by armed groups.
"People fleeing the recent incident claimed that the town came under an indiscriminate attack by the South Sudan armed forces," the statement said.
"Refugees told the UNHCR team on the ground in Lamwo terrifying stories of violence and abuse against civilians. Many have witnessed their loved ones shot dead or slaughtered like animals," UNHCR spokesman Rocco Nuri told AFP.
"Families fled in all directions. Those unable to run were reportedly shot dead, including the elderly and people with disabilities."
A local pastor who fled Parjok on Wednesday, and asked not to be named, said soldiers had entered the town in tanks "and suddenly we saw shooting and we just had to run."
Both he and regional Anglican Bishop Oringa Benard reported that some 135 people had been killed, however this could not be independently verified.
The UN peacekeeping mission UNMISS said in a statement on Wednesday that they had twice been prevented from accessing Pajok.
"The mission has received reports of fighting between SPLA (government) troops and the opposition there and is trying to follow-up on reports of civilians killed in the area," the statement added.
UNMISS urged the government to immediately allow it access "so it can fully implement its mandate, including to protect civilians and report on human rights violations."
South Sudan government spokesman Michael Makuei confirmed that its forces had attacked Pajok.
"As you know Pajok has been in the hands of the rebels so what happened was government forces went there and fighting ensued and so the civilian population that has been staying with the rebels had to run away," he said.
"I don't know the figure (of those who might have been killed or injured) but in any fighting there must be casualties."
Uganda currently hosts more than 832,000 refugees from South Sudan, including over 270,000 in the Bidibidi refugee camp which in eight months has gone from an empty patch of land to the world's biggest refugee camp.