The U.S. official urged the military force to refrain from violence when handling the protesters.
Soldiers killed 114 civilians, raped countless women and attacked funerals in a recent offensive, according to a U.N. report.
At least 82 aid workers have been killed since 2013 in South Sudan, the U.N. says.
Nearly half the population will not have a reliable food source by summer.
Western countries should follow the example of Uganda, an aid agency has said.
The Security Council has failed to impose an arms embargo on the conflict-hit state.
A U.N. human rights commission said that ethnic cleansing was already underway in South Sudan.
South Sudan's crisis is getting worse, and international bodies are standing idly by.
European and Middle Eastern firms use African middlemen to sell arms to the war-torn state.
Peace will elude South Sudan as long as its women's rights and safety continue to be ignored and abused.
Report accused top officials, including Salva Kiir and Riek Machar, of benefiting from the conflict.
Top South Sudanese officials have accrued fortunes during the country's civil war.
Machar fled South Sudan after fighting in the capital Juba in July.
The deposed vice-president fled South Sudan earlier in August.
Kerry met with East African ministers on Monday to discuss possible solutions to the conflict.
Some 16,000 children have been recruited to fight in South Sudan since December 2013.
The rampage in Juba did not see UN troops, stationed only a mile away, intervene.
The government wants an independent African force to calm tensions in the capital Juba.
Rebel leader Riek Machar has still not returned to Juba after clashes in July.
The world's youngest country needs outside intervention to avoid all-out war.
At least 120 cases of sexual violence and rape have been reported in the last three weeks.