The European Union named two South Sudanese military leaders who will face sanctions: rebel leader Peter Gadet and government army commander Santino Deng.
They will face travel bans to EU countries as well as asset freezes.
Gadet commanded an ethnic militia that massacred more than 200 civilians in April, during an offensive on an oil town in the country's northern Unity State, the EU said.
His militia committed a series of human rights abuses in an assault on Bentiu town in April, and also violated a January ceasefire agreed by rebel and government leaders.
"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.
Deng was targeted as he participated in the recapture of the same town.
"Santino Deng is thus responsible for the violations of the 23 January Cessation of Hostilities Agreement," the EU said.
South Sudan's political tensions erupted in December after President Salva Kiir accused his deputy, Riek Machar of plotting a coup. Machar subsequently launched attacks on government forces, first in Juba and later across the country.
The two leaders reached to a ceasefire in January but hostilities resumed within hours of the agreement. The two sides met in May to agree a new truce although talks have come to a halt.
President Kiir's spokesman said the sanctions could hamper peace talks between the two sides.
"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," Kiir's spokesman Ateny Wek Ateny said, as quoted by Reuters.
The United Nations estimates that 10,000 people have been killed during the conflict in the world's newest country, while around 1 million South Sudanese have been displaced.
South Sudan gained independence from its northern neighbour in 2011 but it has been beset by inter-ethnic rivalries and fighting in 2014.