The European Union said on Friday it was imposing sanctions on Peter Gadet, a South Sudanese rebel leader who commanded an ethnic militia that massacred more than 200 civilians in April.
The EU announced travel bans and asset freezes on two South Sudanese military leaders on Thursday but did not name them at the time. The other officer named on Friday was Santino Deng, a commander of the third Infantry Division of the government army.
The EU said Gadet had been in charge of a militia during a three-day assault on an oil town in northern Unity State.
Fighting erupted in Juba in December between the government forces of President Salva Kiir and supporters of Riek Machar, his former deputy and long-time rival. The conflict has revived deep ethnic tensions in the world's newest country, which won independence from Sudan only in 2011.
Peace talks between Kiir and Machar stalled after they last met in May and agreed a truce. A previous one agreed in January quickly collapsed. The U.N. estimates more than 10,000 people have been killed since December, and about a million displaced.
In May, the United States placed sanctions on Gadet and Major-General Marial Chanuong, head of Kiir's presidential guard, freezing any assets they might hold in the United States, and blocked U.S. citizens or companies from dealing with them.
"Peter Gadet is thus responsible for fuelling the cycle of violence, thus obstructing the political process in South Sudan, and for serious human rights violations," the EU said in its official journal.
It also imposed sanctions on Deng, a commander of the third Infantry Division of the government army, as he participated in the recapture of Bentiu.
"Santino Deng is thus responsible for violations of the 23 January Cessation of Hostilities Agreement," the EU said.
Gadet and Deng were not immediately reachable for comment.
Ateny Wek Ateny, the spokesman for Kiir, said the sanctions could be a "setback to the peace talks".
"The government was defending the constitution. There is no government in the world where the army won't fight when someone wanted to overthrow the constitution," Ateny told Reuters by phone.
Ateny added that government officials were in a meeting to make decisions on how to respond to EU sanctions.
In a statement issued on Thursday before it was revealed Deng and Gadet were the sanctioned commanders, the rebels said they welcomed the sanctions, but added the punishments should have been more focused.
"The (opposition) encourages the international community to impose more targeted sanctions that can have a greater effect in compelling the parties to negotiate in good faith," Mabior Garang de Mabior, its spokesman, said in the statement.
Last month, IGAD threatened to also impose sanctions on the warring sides unless they stopped all military operations.
(Additional reporting by George Obulutsa in Nairobi, Aaron Maasho in Addis Ababa, Julia Fioretti in Brussels; Editing by Drazen Jorgic)