United States:
UN chief Ban Ki-moon called on South Sudan's warring leaders on Thursday to immediately stop fighting following the latest upsurge of violence in the oil-rich north.
The UN secretary general "strongly condemns" the resumption of fighting and urges the sides to "reach urgently" an agreement on a political transition, said a statement from his spokesman.
Ban "calls upon President Salva Kiir and Dr. Riek Machar to cease immediately all military operations and reminds them of their obligation to protect civilians and respect international humanitarian law," he said.
The fresh fighting in the key northern oil town of Bentiu marked an end to a brief lull in hostilities in the country's 10-month war and coincides with the end of the rainy season, which made many roads impassable.
Ban warned the sides of the "inviolability" of UN bases in South Sudan, where 100,000 civilians are sheltering, fearing they will be killed if they leave.
The United Nations has been working with regional leaders of the IGAD group to broker a peace deal and has threatened to impose sanctions on those who reject a settlement.
South Sudan descended into chaos and violence in December when a political dispute broke out between Kiir and his former vice president Machar, but the turmoil has since broadened into an ethnic conflict.
The humanitarian situation in the country remains dire, with 1.8 million people displaced including 450,000 to neighbouring countries.
Around four million people -- close to a third of the population -- are facing a food crisis.