JUBA (AFP) -- Rebel gunmen in South Sudan massacred "hundreds" of civilians in ethnic killings when they captured the oil town of Bentiu last week, the UN said Monday, one of the worst reported atrocities in the war-torn nation.

In the main mosque alone, "more than 200 civilians were reportedly killed and over 400 wounded," the UN mission in the country said, adding there were also massacres at a church, hospital and an abandoned UN World Food Programme compound.

Fighters took to the radio urging rival groups to be forced from the town and for men to rape women from the opposition ethnic group.

South Sudan's army has been fighting rebels loyal to sacked vice president Riek Machar, who launched a renewed offensive this month targeting key oil fields.

The conflict has also taken on an ethnic dimension, pitting President Salva Kiir's Dinka tribe against militia forces from Machar's Nuer people.

UN human rights investigators said that after rebels wrested Bentiu from government forces in heavy battles last Tuesday, the gunmen spent two days hunting down those they believed opposed them.

Both South Sudanese and Sudanese -- some from the war-torn Darfur region -- were killed, the UN peacekeeping mission in South Sudan (UNMISS) said in a statement.

Hate radio urged rape

"They (the rebels) 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," the UN said.

Some rebels took to the local radio to "broadcast hate messages declaring that certain ethnic groups should not stay in Bentiu, and even calling on men from one community to commit vengeful sexual violence against women from another community," the statement added.

Rebel spokesman Lul Ruai Koang, in a statement released shortly after the capture of the town, had praised the "gallant forces" of having completed "mopping and cleaning up operations in and around Bentiu".

At the Kali-Ballee mosque, where hundreds had taken shelter, the rebels "separated individuals of certain nationalities and ethnic groups and escorted them to safety, while the others were killed," the UN report said.

At the hospital, "several Nuer men, women and children were killed for hiding and declining to join other Nuers who had gone out to cheer" the rebels as they entered the town, the UN said.

Peacekeepers later rescued over 500 civilians, many of them wounded, from the hospital and other sites, as well as guarding "thousands" of civilians as they walked to the UN base, where over 12,000 people are now crammed in for shelter.

Similar killings were reported at the Catholic church and WFP compound.

Peacekeepers have already said they had seen dozens of corpses in military uniform on the streets of the northern town, capital of Unity state.

Rebels say 'mopping up'

The capture of Bentiu came two days before gunmen stormed a UN compound in which at least 58 people were killed, with peacekeepers fighting back to protect over 5,000 civilians sheltering there who the attackers had wanted to kill.

The UN Security Council said that attack may "constitute a war crime".

The surge in fighting in the four-month-long conflict comes amid warnings by UN Secretary General Ban Ki-moon that more than one million people are at risk of famine in the war-torn country.

A ceasefire deal in tatters, while peace talks in luxury hotels in Ethiopia have made little if any progress.

Bentiu is the first major settlement to have been retaken in a renewed offensive by Machar forces, with the rebels saying on Monday that fighting continued in Unity state and boasting of further villages they had captured.

They could not be contacted for comment on the reports of massacres.

The conflict in South Sudan, which only won independence from Sudan in 2011 and is the world's youngest nation, has left thousands dead and forced around a million people to flee their homes.

The fighting has been marked by reports and allegations of atrocities by both sides, with ethnic massacres, child soldier recruitment and patients raped and murdered in hospitals by attacking forces.

"These atrocities must be fully investigated and the perpetrators and their commanders shall be held accountable", Raisedon Zenenga, deputy head of UNMISS.

The United States, the key backer of South Sudan's move to independence less than three years ago, has threatened targeted sanctions against those responsible for the violence.