The United States is imposing visa restrictions on members of the government and others who have obstructed humanitarian aid to the country by taxing shipments, the State Department said on Friday.

South Sudan, where hundreds of thousands of people died as a result of civil war from 2013-2018, is experiencing one of the world's worst humanitarian crises as a result of persistent conflict, natural disasters and poverty.

UN missions there have said South Sudanese authorities are holding up United Nations fuel tankers over a tax dispute, jeopardising the delivery of millions of dollars of aid during a humanitarian crisis.

"Despite assurances, the government has yet to effectively reduce the unacceptably high costs, bureaucratic obstacles, and risks of providing humanitarian assistance to South Sudanese people in need," State Department spokesperson Matthew Miller said.

"This raises questions about its willingness and capacity to abide by its 2018 peace agreement commitment to create an enabling environment for the delivery of humanitarian assistance and protection," he said in a statement.

The visa restrictions would make those cited ineligible entry into the United States.