Human Rights Watch on Thursday called for an international probe into what it said were serious abuses carried out by both government and rebel forces in South Sudan's month-old conflict.
The group also said the United Nations should impose an asset freeze and travel ban on anyone in the country linked to violations of human rights and humanitarian laws.
"Appalling crimes have been committed against civilians for no other reason than their ethnicity," said Daniel Bekele, Africa director at Human Rights Watch.
"Both sides need to leave civilians out of their conflict, let aid groups reach people who need help and accept a credible, independent investigation into these crimes," he said in a statement.
Fighting erupted in South Sudan on December 15 following a clash between army units loyal to President Salva Kiir and his main rival, former vice president Riek Machar, escalating into all-out war and sparking ethnic reprisals between members of Kiir's majority Dinka tribe and Machar's Nuer group.
South Sudan is the world's youngest nation which gained independence from Khartoum less than three years ago.
Human Rights Watch said its researchers have interviewed more than 200 victims and witnesses to abuses in the capital Juba and the town of Bor, capital of Jonglei state and the scene of constant heavy fighting since the conflict began.
It said it has "documented widespread killings of Nuer men by members of South Sudanese armed forces in Juba," including a massacre of between 200 and 300 men in the Gudele neighbourhood on December 16 -- a massacre that was also documented and reported on by AFP.
"Researchers also documented the targeting and killing of civilians of Dinka ethnicity by opposition forces in other parts of the country," the group said, adding that "many of the crimes committed are serious violations of international humanitarian law and may constitute war crimes or crimes against humanity."
It said the UN, which has peacekeepers in South Sudan, needed to accelerate the deployment of reinforcements and take "urgent steps to improve the protection of civilians, including better security around UNMISS compounds sheltering some 66,500 civilians displaced by conflict."
It also condemned "looting of medical and humanitarian facilities" as well as "government denials of flight authorisation to areas where people are in desperate need of aid."