Washington - Washington has warned that those who perpetrate atrocities in South Sudan's civil war will be held responsible for their crimes.The United States served as a midwife in the creation of the South Sudan, formed in July 2011 by partitioning Sudan.But South Sudan descended into war in December 2013, and a peace deal signed last year collapsed during heavy fighting in the capital this month."Recent weeks have featured well-documented reports of civilian killings and a surge in the number of government soldiers in uniform raping and gang raping women and girls who have taken refuge in UN Protection of Civilian sites," the US State Department said late on Saturday.The United Nations "has documented at least 120 cases of sexual violence in the last two weeks" in fighting between government forces and those loyal to rebel chief Riek Machar."Those responsible for war crimes, crimes against humanity, and other violations of international humanitarian law - including those who order or incite violence, or encourage or contribute to the commission of crimes - will be held accountable," the US statement read.The statement reminded "all parties" that the peace agreement provides for a special court that "will have jurisdiction over violations of international law committed during the transitional period, including those committed during the ongoing violence."Washington also called for "an immediate halt to combat operations and full compliance with the ceasefire declared on July 11 and in the peace agreement"Edge of an abyssJuba was rocked by several days of heavy fighting in early July between government forces and those loyal to Machar, in the latest upsurge in the two-and-half-year war.Nearly 300 people died in the violence and two Chinese peacekeepers were killed in an attack on a UN base, where thousands of civilians rushed for safety.South Sudan is "poised on the brink of an abyss" UN Secretary General Ban Ki-moon told the Security Council on Thursday. Ban has called for an arms embargo and sanctions to be imposed on those who fail to observe the peace agreement in South Sudan, but the council has yet to endorse his call. In 2015, the UN and the US imposed sanctions on the main players in the conflict, with the State Department reiterating its warning that "those taking actions threatening the peace, security, or stability of South Sudan... may be subject to sanctions under UN Security Council Resolution 2206 ".