President Barack Obama's national security adviser Susan Rice urged warring leaders in South Sudan on Thursday to end their conflict, blaming the nation's president, Salva Kiir, and rebel leader Riek Machar for "appalling crimes" and violence.

In an impassioned video message marking the fourth anniversary of South Sudan's Independence Day, Rice called on parties to form a transitional government and said "it breaks my heart to see what South Sudan has become."

"The government and rebels are committing appalling crimes against innocent women, children, and the elderly. President Kiir and Riek Machar and their cronies are personally responsible for this new war and self-inflicted disaster," Rice said.

In her message, Rice recalled how she and her teenage son went to Juba, South Sudan's capital, four years ago when the country won its independence from Sudan and ended a decades-long civil war.

"I remember the hope and unity of that day," she said.

The nation plunged into civil war in December 2013. Thousands of people have been killed and more than 2.2 million have fled their homes.

"President Kiir and Riek Machar would rather haggle over personal power and wealth than agree on solutions," she said.

"Over the past 19 months, the government has abdicated its responsibilities, failed to protect its citizens, and squandered its legitimacy.

"Instead of negotiating a resolution to the conflict, it has subverted democracy and unilaterally extended its mandate," she said, noting the conflict threatens to destabilize the region.

Rice said the United States would support regional efforts to try to end the conflict and would hold those accountable those responsible for violence.

"The United States, along with the international community, will punish those determined to drive South Sudan into the abyss," Rice said.