The U.N. chief is strongly condemning the commandeering of humanitarian vehicles and the theft of food and other desperately needed aid by government and anti-government forces in violence-torn South Sudan.

Secretary-General Ban Ki-moon also expressed alarm Wednesday at the rising death toll from fighting in the world's newest nation.

Ban is expressing concern at the rising number of people displaced by the fighting, which surpassed 400,000 this week, U.N. spokesman Martin Nesirky says.

The secretary-general calls on all parties to stop the violence and engage in negotiations led by a bloc of East African countries known as IGAD, Nesirky says.

The spokesman says Ban reiterates that those responsible for attacks against civilians, humanitarian workers and U.N. personnel will be held accountable.

The violence in the oil-rich country began in mid-December.