UN says rebels slaughtered hundreds of people when they seized capital of Unity state, urged on by hate speech on radio

The UN's top humanitarian official said on Tuesday that a massacre in South Sudan fuelled by hate speech on a public radio station was a "game-changer" in the country's four-month conflict.

Toby Lanzer said he saw "piles and piles" of bodies on a visit to the oil hub of Bentiu after rebels wiped out civilians based on their ethnicity and nationality. TV pictures showed corpses lying outside a mosque and piled up on a mechanical digger.

It is probably the worst single atrocity since fighting broke out in the world's newest country last December, and raises the prospect of a full-blown civil war along ethnic lines, intensifying pressure on the international community to intervene.

According to the UN, rebels slaughtered hundreds of people when they seized Bentiu, the capital of Unity state, hunting down men, women and children who had sought refuge in a hospital, a mosque and a Catholic church. The victims included Sudanese traders, especially from the Darfur region.

The rebels issued a statement boasting of "mopping- and cleaning-up operations", the UN alleged, and fighters took to the radio to broadcast hate speech, urging men to rape women of specific ethnicities and demanding that rival groups be expelled from the town.

"Use of hate speech via a public radio station to incite violence is a game-changer," said Lanzer, who was in Bentiu on Sunday and Monday.

He said thousands of civilians were now streaming to the UN base in Bentiu fearing that more violence was imminent. The cramped base holds 25,000 people and has very little water and only one latrine per 350 people.

A rebel spokesman, Lul Ruai Koang, dismissed the UN accusations as "unfounded, cheap propaganda" and put the blame on government forces.

What began as a political power struggle in South Sudan quickly assumed an ethnic dimension, pitting President Salva Kiir's Dinka tribe against militia forces from the former vice-president Riek Machar's Nuer people. Peace talks have failed to stem the flow of atrocities on both sides.

Donatella Rovera, a senior crisis response adviser at Amnesty International, who has just returned from the country, said: "Whatever the reasons this conflict broke out on 15 December – and there were obviously political grievances – from the first day it has taken on markedly an ethnic dimension. We saw this very clearly with the attack by opposition forces on Bentiu."

She said there was worse to come in terms of food shortages. "We're seeing two big killers in South Sudan. One is people being killed along ethnic lines. The other, that will kill even more people, is the unfolding humanitarian catastrophe. It's the planting season and no one is able to plant. This is going to be a larger catastrophe still and it is entirely down to the conflict."

The fighting has left thousands dead and forced around a million people to flee their homes. The government says its forces are currently battling rebels in three key states as Machar's fighters continue an offensive targeting oil fields. Peace talks are due to restart in neighbouring Ethiopia this month.

Oxfam International called for a ceasefire and a "massive injection" of emergency aid. Winnie Byanyima, its executive director, said: "How many lives have to be lost before the parties to the conflict silence their guns and donors responds with more resources? We either act now or face an even larger human catastrophe in the weeks and months to come."

The United States has threatened sanctions against those responsible for fuelling the conflict. Samantha Power, the US ambassador to the UN, said: "All responsible for South Sudan horrors and deliberately targeting civilians must be held accountable."