The UN refugee agency said now around 100,000 people - many of whom had fled into the town in search of safety - could no longer leave after government troops surrounded the area.
The United Nations warned on Friday that around 100,000 people were trapped in the South Sudanese town of Yei, where they were facing serious shortages of food and medicine.
Yei is situated some 150 kilometres southwest of Juba, near the borders with Uganda and the Democratic Republic of Congo, and until recently had been spared much of the violence plaguing the world's youngest nation. But the security situation there has deteriorated rapidly since July.
"The government forces are surrounding the town and they are restricting access into the town and also preventing people from leaving, presumably because they suspect them of siding with opposition forces," UNHCR spokesman William Spindler told reporters.
He pointed out that more than 30,000 people had fled into Yei from surrounding areas following deadly attacks on civilians and looting of private property earlier this month.
They joined several thousand displaced people who had arrived since mid-July, and as many as 60,000 town residents, he said.
UNHCR and other UN agencies had visited the town on September 27, discovering that tens of thousands of displaced people had taken refuge in abandoned houses and church compounds, Spindler said.
"Terrorised men and women spoke of horrific violence against civilians before and during their flight," the refugee agency said, pointing to killings, mutilations and the looting and burning of property.
In Yei, the displaced "are facing a serious shortage of food and medicine," Spindler said.
He said humanitarian workers were hoping to soon bring desperately needed aid into the town, but acknowledged that the timeframe for such a delivery remained unclear.
South Sudan, which gained independence in July 2011, descended into war just two and a half years later when President Salva Kiir in December 2013 accused his former deputy Riek Machar of plotting a coup.
Numerous attempts to shore up a fragile truce have failed, and in a major setback to peace efforts, fresh clashes erupted in Juba on July 8 this year between Kiir's guards and troops loyal to Machar.
Since the fresh violence in July, more than 200,000 people have fled South Sudan, sending the number of refugees from the war-scarred nation past the one-million mark, UNHCR said. Another 1.61 million people are displaced inside the country, it said.
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