As many as 129 children were murdered in just one state in South Sudan during fighting in May, including numerous girls who were raped before being slain, and boys who were castrated and left to bleed to death.

The UN's Children's Rights and Emergency Relief Organization (UNICEF) said that the atrocities took place in Unity State, and that they were likely carried out at least in part by government troops and allied groups. UNICEF staff interviewed internally displaced civilians who had fled to the UN base in Bentiu, the Unity State capital, where more than 72,600 South Sudanese are currently seeking shelter.

The use of children as soldiers in South Sudan is endemic, but witnesses said that the victims who were recently killed were all among civilian populations. Some of the slain reportedly included newborns.

Christopher Tidey, a communications officer at UNICEF, told VICE News that witnesses implicated perpetrators belonging to the government Sudan People's Liberation Army (SPLA) and the allied South Sudan Liberation Army (SSLA), who launched an offensive in Unity State in late April.

In at least one case, a girl was "killed because the attackers could not decide on who would rape the victim first," said Tidey. "So the child was killed to resolve the dispute." Other girls, he said, were "abducted to be used as wives."

South Sudan's civil war began in December 2013 in Juba, the country's capital, when fighting broke out between the forces of President Salva Kiir and rebels led by Riek Machar, Kiir's former vice president. Kiir is an ethnic Dinka while Machar is a Nuer, and fighting has largely split along ethnic lines. Prior to the latest reports from Unity State, both sides in the conflict had been accused repeatedly of gross human rights violations and war crimes.

Witnesses who had arrived at the Bentiu camp said that some boys had their genitals cut off and were left to bleed to death. They recounted how entire families were thrown into burning houses, and girls as young as eight were raped before being murdered.

"In one incident, [boys] were tied up and had their throats slit," said Tidey. "It seems that in a lot of these cases the children were targeted. The logic is that they don't want the children to grow up to be the next generation who will then pick up the fight or exact some sort of revenge."

UNICEF said that during fighting in May in Unity State, at least dozens of children who survived massacres were recruited to fight by armed groups.

On June 1, the UN announced that the government of South Sudan had expelled its humanitarian coordinator in the country, Toby Lanzer, in a move that many tied to his comments about the government offensive in Unity State. On May 18, Lanzer said eyewitnesses had reported that operations in the states of both Unity and Upper Nile included "targeted rape and killings of civilians including children."

Lanzer, who also served as deputy special representative at the UN Mission in South Sudan (UNMISS), did not directly implicate government troops. However, just three days earlier, the Intergovernmental Authority on Development, a regional trade bloc that has been overseeing a fruitless peace negotiation process, said its monitors reported that "government forces have been conducting full-scale military offensive against opposition forces in Rubkona, Mayom, Guit, Koch and Mayendit counties in Unity State since 27 April 2015."

Eyewitnesses that UNICEF spoke with had fled from Rubkona, Guit, Koch, and Leer provinces in Unity State during May, according to Tidey.

On Wednesday, UN peacekeeping chief Herve Ladsous heavily criticized the government of South Sudan for interfering with UNMISS operations in South Sudan. Speaking before the Security Council, Ladsous said that the government in Juba had repeatedly restricted the movement of blue helmets in the country, and that it had denied requests for attack helicopters and the use of unmanned aerial vehicles.

"Juba did declare some of our senior personnel persona non grata," he said. "If you look at that fact that yesterday it was announced that UN personnel taking pictures will be considered a spy, I think this raises a number of concerns."

UNMISS currently maintains a force of some 12,000 peacekeepers. They were first sent in large part to help secure national development in the world's youngest country, but are now largely confined to safeguarding the more than 136,600 South Sudanese that have sought shelter at their bases. Yet even there, civilians have been sporadically killed during heavy fighting.

The alleged atrocities in Unity State last month are only the latest in a conflict that has become a hallmark for sexual violence and attacks against children. In January, Zainab Bangura, the UN special representative on sexual violence in conflict**,** said that survivors and healthcare workers had recounted to her "heartbreaking stories of rape, gang rape, abductions, sexual slavery, and forced marriage." One hospital in Juba, she told reporters, had said that 74 percent of rape and sexual assault victims were minors. The youngest victim was two years old.

Last April, a rebel assault on Bentiu saw the execution of hundreds of civilians. During fighting, rebels commandeered a radio station from which they broadcast messages that "called on men to avenge past gender-based violence against women of our community," according to UNMISS. The armed groups also reportedly used a local church as a "rape camp" for several months.

As news of the atrocities in Bentiu began to emerge, many observers saw the event as a turning point in the conflict. Yet a year later, witnesses who fled to Bentiu reported similar stories.

While the Security Council has strongly condemned the bloodshed in South Sudan — and in recent months its government in particular — it has yet to make use of the sanctions mechanism it established for individuals in the country back in March. Council diplomats say that the process is being held up by the American government, as it decides which names should be put before the council.

On Thursday, Farhan Haq, spokesperson for Secretary-General Ban Ki-moon, said that a human rights report on the violations and abuses that had taken place in Unity State would soon be released to the public.

In a statement, UNICEF Director Anthony Lake said that an estimated 13,000 children had been forced to fight in South Sudan's civil war. "In the name of humanity and common decency, this violence against the innocent must stop."