After rebel forces took control of the oil town last Tuesday, they killed hundreds of civilians after they had determined their ethnicity, according to a report from US peacekeepers on the ground.
"Norway strongly condemns the massacres in southern Sudan," Norwegian foreign minister Børge Brende said. "This must be investigated and the guilty must be punished.
"Women, men and children were slaughtered in large numbers, even in hospitals and religious buildings," he added. "I repeat that both parties to the conflict have a responsibility to stop the violence and resolve their differences through politics and not by killing civilians to spread terror and unrest."
According to a UN report, Nuer rebels broke into a mosque in the town on Tuesday and slaughtered some 200 people inside.
They also murdered civilians sheltering in a Roman Catholic Church and in a hospital.
The United Nations mission in South Sudan has condemned the attacks as "the targeted killings of civilians based on their ethnic origins and nationality".
"They searched a number of places where hundreds of South Sudanese and foreign civilians had taken refuge and killed hundreds of the civilians after determining their ethnicity or nationality", according to a UN statement.
Rebel leader Riek Machar on Tuesday said that his focus were not behind the massacre.
"I contacted the field military commander in Bentiu who told me that such accusation is false," he told Al Jazeera. "First of all we respect our people, and the majority of the forces are from the region and we can't kill our own citizens."
Machar, the leader of South Sudan's Nuer people, was the country's vice President until his dismissal last July.
Since a failed coup d'etat against South Sudan's president Salva Kiir last December, Machar has been described as the leader of the rebels in the country.