NAIROBI, Kenya?- The removal of South Sudan's military chief of staff - who comes from the same ethnic group as rebels fighting the government?- further freezes a powerful minority out of top government positions, an analyst said Thursday, as battles continued following a massacre by rebels.

President Salva Kiir replaced Gen. James Hoth Mai, an ethnic Nuer, who served as the military's top leader since 2009. Military spokesman Col. Philip Aguer said Thursday that Mai's removal is the second reshuffling of the military in nearly a decade.

Mai comes from the same ethnic group as the majority of the rebel fighters in the country, who are led by the country's former vice president, Riek Machar, a Nuer. Mai's position was frequently cited as an example of the ethnic diversity of the government led by Kiir, an ethnic Dinka.

Kiir also relieved his director of military intelligence following that official's testimony at a recent treason trial that appeared to politicize the army, said Casie Copeland, a South Sudan analyst for the International Crisis Group.

Mai's firing will make the Nuers who remain in government nervous, she said.

"The removal of the chief of staff and director of military intelligence are indicative of the government's increasing tendency to place those deemed to be sufficiently loyal to the president in key positions," she said.

The newly named replacements in the two leadership positions are Dinka, like the president, she said.

Aguer, the military spokesman, noted that chiefs of staff are not for life, and that the military implements the decisions of the president. It is the second change in the top military position since 2005, Aguer said. He declined to comment on whether the change of ethnicities in the leadership would affect army morale.

"Anybody can interpret anything the way he see it, but me, I'm not here to analyze. I'm here to implement," he said.

Aguer reported battles in Jonglei state on Wednesday around the town of Renk, which he said is in the military's control.

The situation in Bentiu?- where hundreds of people were slaughtered by rebel forces last week?- remains calm but in rebel control, he said.

The U.N. Security Council on Wednesday viewed "horrific pictures of corpses" from the scene of last week's massacre in Bentiu and discussed taking actions that could include sanctions, diplomats said.

In a tweet after the meeting, U.S. Ambassador Samantha Power called for sanctions on "political spoilers and those who target civilians." President Barack Obama earlier this month warned that the United States may levy sanctions, including visa bans and asset freezes, on individuals and entities involved in stoking violence in South Sudan.

Violence has been raging in South Sudan since mid-December, when Kiir accused Machar of trying to orchestrate a coup.