SPLA soldiers reportedly arrested eight Nuer civilians during house-to-house searches in Juba's Munuki area and took them to two nearby hotels, "where they shot four of them".
Government soldiers and security forces in South Sudan executed civilians and gang-raped women and girls during and after ethnically-charged fighting last month in the capital Juba, the United Nations said on Thursday.
Zeid Ra'ad Al Hussein, U.N. High Commissioner for Human Rights, called on the government of President Salva Kiir to prosecute perpetrators and urged world powers in the U.N. Security Council to take "urgent action" to halt violence.
"While some civilians were killed in crossfire between the fighting forces, others were reportedly summarily executed by Government (SPLA) soldiers, who appear to have specifically targeted people of Nuer origin," Zeid said in a statement.
Kiir fired six ministers allied to his long-time rival Riek Machar late on Tuesday, widening a political rift in the world's newest state and drawing threats of more fighting.
Zeid, reporting on his office's investigation, cited two separate incidents on 11 July in which SPLA soldiers reportedly arrested eight Nuer civilians during house-to-house searches in Juba's Munuki area and took them to two nearby hotels, "where they shot four of them".
On the same day, SPLA soldiers broke into another hotel where they shot and killed a Nuer journalist.
At least 73 civilian deaths have been documented so far by the U.N., but it is believed the civilian death toll may turn out to be "much higher", Zeid said.
"The fighting also resulted in widespread sexual violence, including rape and gang rape by soldiers in uniform and men in plain clothes," he said, adding that Nuer, Dinka and women from the three Equatorial states were all targeted, along with foreign nationals.
At least 217 cases of sexual violence in Juba had been documented during the period of July 8-25, he said.
"In a few areas, women from various ethnic groups were raped by heavily armed youth believed to be affiliated to the SPLA in Opposition (SPLA/IO)," Zeid said.
"However, according to the information we have gathered so far, those most affected were displaced Nuer women and girls and those responsible seem to have been mostly SPLA."