The U.S. on March 21 placed sanctions on 15 South Sudanese oil-linked operators it said were substantial sources of revenues for the government, aimed at increasing pressure on President Salva Kiir to end the country's conflict.

"By placing these entities on the U.S. Department of Commerce's Entity List, the United States will impose a license requirement on all exports, re-exports, and transfers of any U.S.-origin items to those entities," the State Department said in a statement.

Those on the list include government, state-linked and private groups in South Sudan that "are involved in activities that are contrary to the foreign policy interests of the United States," according to a related Department of Commerce document published on March 21.

Washington has already placed sanctions on South Sudanese military and political figures, and in January imposed an arms embargo to halt the flow of weapons into the country, signaling that it had lost patience with South Sudan's warring factions for ignoring ceasefires.

Last year, the South Sudanese government said it planned to more than double oil production to 290,000 barrels per day in fiscal 2017/2018.

As well as 13 firms, the Commerce Department placed the South Sudan Ministry of Mining and the South Sudan Ministry of Petroleum on its list.

South Sudan has been wracked by civil war since 2013 when troops loyal to Kiir clashed with troops loyal to then-Vice President Riek Machar.

Since then, the conflict has claimed tens of thousands of lives, slashed oil production and driven about a third of the population of 12 million from their homes.