The United States Agency for International Development (USAid) administrator, Mark Green, has termed South Sudan as the most dangerous country for aid workers but has also expressed hope that the civil war can be stopped.
Mr Green, in his two-day visit to the South Sudanese capital Juba and the northwestern Wau region, also met President Salva Kiir and urged him to end the suffering of the people.
He called on Mr Kiir to restore a permanent ceasefire, end obstruction of humanitarian access, eradicate exorbitant fees levied on aid organisations, and engage the warring parties in an inclusive peace process.
According to the United Nations agency, UNHCR, over one million South Sudanese refugees have fled to Uganda.
An estimated 1.8 million people in South Sudan are receiving humanitarian assistance with support from the US, Mr Green said.
President Kiir has been hoping that the National Dialogue he launched in 2016 would help bring peace after four years of fighting, but the civil war keeps spreading as more ethnic militias emerge.
The United States is the single-largest humanitarian donor to South Sudan, having provided nearly $2.7 billion since the conflict erupted in 2013.
The crisis was ignited after President Salva Kiir, an ethnic Dinka, fired his then deputy Riek Machar, a Nuer.
About one third of the population has fled to neighbouring countries for safety.