- The sanctions are against Defense Minister Kuol Manyang Juuk and Martin Elia Lomuro, minister of cabinet affairs.
- Lomuro was accused of actively recruiting and organizing local militias to conduct attacks against opposition forces in South Sudan.
- The US accused Manyang of failing to remove military forces from the battlefield as agreed and overseeing training of tribal militia.
The United States on Monday imposed sanctions on two sitting ministers in South Sudan, accusing them of obstructing the young country's peace efforts despite promises to form a unity government.
The order against Defense Minister Kuol Manyang Juuk and Martin Elia Lomuro, minister of cabinet affairs, freezes any assets they have in the United States and bans them from entering.
The sanctions "are intended to target senior leaders in South Sudan that have perpetuated the conflict for their own personal enrichment, leading to much suffering for the South Sudanese people," the Treasury Department said in a statement.
The action is the latest by the United States -- a key supporter of the largely Christian nation's independence in 2011 from Arab- and Muslim-dominated Sudan -- to show impatience over a leadership battle that has fuelled fighting that has killed nearly 400,000 people.
Under a peace deal, President Salva Kiir and his former deputy Riek Machar had agreed to form a government by last month but African mediators gave them a 100-day delay, the second such extension.
More than 30 days into the extended period, the United States said it has "yet to see concrete steps" by South Sudan to create the conditions to form a unity government and fully implement the peace accord.
Lomuro "has been responsible for actively recruiting and organizing local militias to conduct attacks against opposition forces in South Sudan," the Treasury Department said.
For his part, the defence minister "has failed to remove military forces from the battlefield as agreed, fomented violence with rival tribes, and oversaw the training of tribal militias to prepare for the possibility of renewed violence," it said.
The conflict has displaced four million -- one-third of the country's population.
Kiir and Machar held a rare, brief meeting last week in the capital Juba, but US officials fear that the two have grown comfortable with the status quo, with international donors feeding their people.
The United States has also recalled its ambassador for consultations but has indicated it will not curb its roughly $1 billion in annual aid, which largely goes to food and other humanitarian needs.