JUBA (AFP) ― Rebel gunmen in South Sudan massacred “hundreds” of civilians because of their ethnicity when they captured a key oil town last week, the U.N. said Monday, calling for a probe into 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 U.N. mission in the country said. Civilians including children were also massacred at a church, hospital and an abandoned U.N. World Food Program compound, it said.

Toby Lanzer, the top U.N. aid official in the country, said after visiting the town of Bentiu he had witnessed a “most terrible sight.”

“There are piles of bodies lining the streets where they had been executed, in the market, outside and inside places of worship ... the majority wearing civilian clothes,” he said.

Fighters took to the radio to urge men to rape women from the opposition ethnic group and said rival groups should be forced from the town, the U.N. said.

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 an ethnic dimension, pitting President Salva Kiir’s Dinka tribe against militia forces from Machar’s Nuer people.

U.N. 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 who they believed opposed them.

The U.N. peacekeeping mission in South Sudan “strongly condemns these targeted killings,” U.N. spokesman Stephane Dujarric said in New York.

“The mission calls for these atrocities to be fully investigated and for the perpetrators and their commanders to be held accountable,” he added.

Reminding the government and rebels of their obligation to protect civilians, Dujarric said the mission “calls on them to immediately cease targeting unarmed civilians” and to respect a January ceasefire that has fallen to pieces.

Both South Sudanese and Sudanese -- some from the war-torn Darfur region -- were killed, UNMISS said.

Peacekeepers are photographing those killed to provide documentation before burial, Lanzer said, with video footage shot by U.N. workers showing digger machines loaded with corpses.

“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 U.N. statement said.

Some rebels took to 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,” it added.

Shortly after the town was captured rebel spokesman Lul Ruai Koang praised the “gallant forces” for completing “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 U.N. 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 U.N. said.

Similar killings were reported at the Catholic church and World Food Programme compound.

Peacekeepers later rescued more than 500 civilians, many of them wounded, from the hospital and other sites, and are guarding “thousands” of civilians as they continue to stream towards the U.N. base, where more than 22,000 people are now taking shelter in desperate conditions.

“Those in the camp have less than a liter of water per person per day, and that is simply not enough in the heat of South Sudan,” Lanzer said.

The capture of Bentiu came two days before gunmen stormed a U.N. compound in an attack in which at least 58 people were killed, with peacekeepers fighting back to protect more than 5,000 civilians sheltering there whom the attackers had wanted to kill.

The U.N. Security Council said that the attack on Thursday may “constitute a war crime.”

The surge in fighting in the four-month conflict comes amid warnings by U.N. Secretary General Ban Ki-moon that more than 1 million people are at risk of famine in the war-torn country.

The January ceasefire deal is 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’s forces, with the rebels saying Monday that fighting continued in Unity state.

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

The conflict in South Sudan, which won independence from Sudan only 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.

The United States, the key backer of South Sudan’s independence, has threatened targeted sanctions against those responsible for the violence.