Both government and opposition forces and their allies have committed extraordinary acts of cruelty that amount to war crimes in South Sudan since fighting began in December 2013, says Human Rights Watch.

"South Sudan's death toll in this new war is unknown but thousands of civilians have been killed, homes and markets burned, and bodies left to be eaten by birds and dogs," the August 8 report says.

"Towns and large rural areas in Jonglei, Upper Nile and Unity states have emptied, with an estimated 1.5 million people fleeing their homes, afraid that they will be targeted and killed."

It said the mass displacement, looting and destruction caused by the warring parties, the two biggest ethnic groups in South Sudan, have contributed to the emergency-level food shortages which now face 1.1 million people...with the prospect of famine looming on the horizon."

The Lutheran World Federation on Thursday strongly condemned the targeted killing of aid workers in South Sudan's Maban county, as fighting escalates between militia groups in the country's Upper Nile State.

Speaking on the security situation in Maban county, the director of the LWF's Department for World Service, Rev. Eberhard Hitzler said the deteriorating insecurity has compelled the LWF to evacuate its staff from Upper Nile State.

It has been forced to temporarily suspend the current humanitarian intervention work there.

"We reiterate LWF's strong condemnation of the loss of all civilian lives in the current fighting.

"At the same time, we strongly object the targeted killing of aid workers—the very people who are providing emergency assistance to South Sudanese communities and Sudanese refugees," Hitzler said.

Advocacy Mission of the African Committee of Experts on the Rights and Welfare of the Child said the nearly eight-month conflict has caused greater damage than the 21-year period of the civil war involving what is now South Sudan and its northern neighbor Sudan.

"The present conflict can be characterized as nothing less than a war on the children of South Sudan," said ACERWC expert Julia Sloth-Nielsen.

"We are reliably informed that children are being targeted, deliberately," Julia Sloth-Nielsen told the Sudan Tribune newspaper.

South Sudan war began with a poltical dispute between President Salva Kiir, from the Dinka ethnic group, and former the Vice President Riek Machar, an ethnic Nuer.

The conflict soon turned into an all-out war between the president's Dinka tribe and Machar's Nuer ethnic group.

The clashes have left thousands of South Sudanese dead and have driven 1.5 million from their homes, with 100,000 taking refuge in the United Nations peacekeeping bases and 400,000 fleeing to nearby Ethiopia and Uganda.

"The crimes against civilians in South Sudan over the past months, including ethnic killings, will resonate for decades," said Daniel Bekele, Africa director at Human Rights Watch.

"It is essential for both sides to end the cycle of violence against civilians immediately, and to acknowledge and support the need for justice."

The United Nations Security Council on Friday threatened to impose sanctions on the two opposing factions for their inability to resolve the ongoing conflict, Press TV reported.

In a unanimous statement, the 15-member council urged Kiir and Machar to implement a ceasefire. It said that the two sides pursuing of a military solution are unacceptable.

Kiir and Machar signed a peace deal three months ago to establish a unity government by August 10. There are however no signs of unity happening.