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 South Sudanese deserve leaders who are committed to laying the groundwork for a successful, peaceful political transition," Secretary of State Mike Pompeo 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 fueled 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 not seen any concrete steps by South Sudan to form a unity government and fully implement the peace accord.

"We urge the government of South Sudan and opposition leaders to distance themselves from peace process spoilers and to make the compromises necessary to form an inclusive unity government by the extended deadline," Pompeo said.

Lomuro, the minister for cabinet affairs, "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 defense 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 people -- 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.