The United States threatened on Thursday to reduce its foreign assistance to South Sudan unless Juba lifts what it said were illicit fees on humanitarian shipments.

In an unusually pointed statement entitled "Time to Stop Taking Advantage of the United States," the US Bureau of African Affairs accused South Sudan's government of "imposing exorbitant fees on humanitarian shipments" and "obstructing United Nations (UN) peacekeeping operations".

South Sudan's humanitarian affairs minister did not immediately respond to a request for comment. 

The US, which has carried out rapid and deep cuts to foreign aid this year, is the largest humanitarian donor to South Sudan. The country of 12-million has been ravaged by conflict since winning independence from Sudan in 2011.

Foreign donors have repeatedly objected to attempts by South Sudanese authorities to collect taxes on humanitarian imports. 

"These actions constitute egregious violations of South Sudan's international obligations," the US statement said.

"We call on the transitional government to halt these actions immediately. If it does not, the United States will initiate a comprehensive review of our foreign assistance in South Sudan with the likelihood of making significant reductions," the statement added.

Armed conflict has persisted in much of South Sudan since the end in 2018 of a five-year civil war that killed an estimated 400 000 people. 

UN investigators, however, said in a report in September that corruption by political elites was the biggest driver of a humanitarian crisis in which most South Sudanese are facing crisis levels of hunger.

Juba rejected that conclusion, attributing the country's humanitarian problems to conflict, climate change and disruptions to oil exports caused by the war in neighbouring Sudan.