The United States has announced its first sanctions against South Sudan, where clashes between pro- and anti-government forces have left thousands of people dead since mid-December.
Secretary of State John Kerry said the U.S. is imposing financial penalties on Marial Chanuong, a government military commander, and Peter Gadet, a military leader loyal to former vice president Riek Machar.
During a Tuesday appearance with EU foreign policy chief Catherine Ashton, Kerry said both men are responsible for "perpetrating unthinkable violence against civilians."
"We will do our utmost to prevent South Sudan from plunging back into the violence and despair that tore that country apart for so long," he said.
Ashton said the EU also is considering sanctions against Juba.
"I'm worried that this country is on the brink of what could be a civil war, ethnically motivated," she said. "The prospects for famine and a humanitarian disaster are really looming large now, so we need to work together. We need to work to ensure that the leaders in South Sudan really do take the action that you've identified they need to."
The sanctions will freeze any assets the two men have in the United States. The penalties also will bar U.S. citizens and companies from doing business with them.
Kerry visited South Sudan last week and met with President Salva Kiir, who agreed to attend peace talks his rival, Machar, in Ethiopia.
Earlier Tuesday, United Nations Secretary-General Ban Ki-moon said Machar had also agreed to attend talks in Addis Ababa.
Speaking in South Sudan's capital, Juba, on Tuesday, Ban stopped short of saying Machar would meet with South Sudan's president, who told reporters that he is ready to meet with Machar in the Ethiopian capital.
Ban said Machar cautioned he might not make it to Addis Ababa by Friday, the projected date for the meeting, because of his remote location.
Machar has been in hiding for months as rebel and government forces clash across South Sudan. More than 1.2 million people have been displaced by the fighting and ethnic violence.
The unrest stems from a power dispute between Kiir and Machar, which worsened in December. The two sides signed a cease-fire agreement in January, but fighting has continued.
On Monday, both sides signed an agreement to facilitate the delivery of aid to populations in need, and to consider a "month of tranquility" so people can plant crops and care for livestock.
The U.N. has warned of a possible famine in South Sudan unless people can safely return to their fields. President Kiir said Tuesday that there would be a "serious disaster ... if we do not allow our people to cultivate now."
The U.N. refugee agency reports that 11,000 South Sudanese have crossed the border into Ethiopia since Saturday, fleeing clashes between government and rebel troops in the Upper Nile region.
The agency says 315,000 South Sudanese overall have fled to neighboring countries since violence erupted in December, while more than 920,000 others are displaced internally.
Tens of thousands are sheltering at U.N. bases across the country.