UN peacekeepers in South Sudan say they have twice been prevented from accessing the town of Pajok, where the army is accused of killing civilians.
The peacekeepers say they are concerned about reports of fighting and attacks on civilians. They are calling to be allowed to visit the area in the south of the country.
Several thousand people have fled Pajok into neighbouring Uganda.
The army has denied targeting civilians and blamed the killings on bandits.
Some of the refugees said government troops shot their relatives as they ran, or arrested and then killed them.
Refugees in Uganda say soldiers arrived in Pajok on Sunday and began opening fire on residents at random.
One told the BBC there had been beheadings, children had been attacked and looting had taken place.
Other reports said some residents had their throats slit before their bodies were strung up from door frames.
A South Sudanese rebel group, the National Salvation Front, has called for an investigation.
South Sudan’s civil war has caused more than 2.2m people to flee their homes.
The UN has said that ethnic cleansing is taking place and has warned of a possible genocide.