A group of South Sudanese chant at a demonstration held in support of President Salva Kiir and against "foreign encroachment" in Juba, South Sudan Monday, March 10, 2014. Arabic on placard reads "No foreigners are allowed to intervene". The rally participants marched through the city on Monday and shouted slogans of support for President Salva Kiir while also denouncing Hilde Johnson, the top U.N. official in the country, accusing Johnson of supplying arms to rebels fighting South Sudan's government. (AP Photo/Mackenzie Knowles-Coursin)

(Recasts, adds quotes from U.N. officials and U.S. and Russian diplomats, details)

By Michelle Nichols

UNITED NATIONS, May 2 (Reuters) - Ethnic violence in South Sudan risks spiraling into genocide, with the country's leaders locked in a personal struggle for power, top U.N. officials said on Friday as a U.N. Security Council showdown looms over calls to impose targeted sanctions.

Thousands of people have been killed and more than 1 million have fled their homes since fighting erupted in the world's newest nation in December between troops backing President Salva Kiir and soldiers loyal to his sacked deputy, Riek Machar.

The fighting has exacerbated ethnic tensions between Kiir's Dinka people and Machar's Nuer.

Adama Dieng, U.N. special adviser on prevention of genocide, and U.N. Human Rights chief Navi Pillay briefed the 15-member council on Friday. Dieng said the ethnic slaughter of hundred of civilians in Bentui and an attack on a U.N. peacekeeping base in Bor last month had changed the course of the conflict.

"If such attacks are not immediately halted it could plunge the country into serious violence that could spiral out of control," said Dieng, who has visited South Sudan with Pillay. "In the current situation, we see elements that we could categorize as risk factors of genocide and other atrocity crimes."

U.S. Secretary of State John Kerry warned on Thursday that the conflict could descend into genocide. He visited Juba on Friday and urged Kiir and Machar to urgently meet for face-to-face peace talks.

Pillay said she feared that South Sudan's leaders were "locked in a purely personal power struggle, with little or no regard for the appalling suffering that it inflicts on their people."

During their visit, Pillay and Dieng warned those leaders they would "inevitably be the subject of international investigations regarding the extent of their knowledge of war crimes and crimes against humanity committed by subordinates under their authority and their failure to take reasonable measures to prevent such crimes."

Several Security Council members called for the situation in South Sudan to be referred to the International Criminal Court.

USA VERSUS RUSSIA

U.S. Ambassador to the United Nations Samantha Power urged the council on Friday to consider imposing targeted sanctions in parallel with such action by the United States in a bid to stop the "outrageous attacks" on civilians and the U.N. peacekeeping mission in South Sudan, which is known as UNMISS.

"In the coming days my government will join in circulating a resolution that will revise the mandate of UNMISS to focus more fully on civilian protection, human rights monitoring and investigation and the delivery of food and other emergency supplies," Power said.

"This council should take up that resolution with the urgency that this crisis demands," she said.

The United States and the European Union have threatened South Sudan with sanctions. President Barack Obama last month authorized possible targeted sanctions against those committing human rights abuses in South Sudan or undermining democracy and obstructing the peace process.

The Security Council needs to renew the mandate for the U.N. mission by July. In December, it approved almost doubling the number of peacekeepers to 12,500 troops as violence worsened but so far less than half of those 5,500 reinforcements have arrived.

Russia's deputy U.N. ambassador, Alexander Pankin, said the calls for targeted sanctions should be addressed "extremely warily and cautiously" and that the opinions of leading regional actors on such an initiative should be canvassed.

"Our extensive, collective experience shows that sanctions are far from a panacea and never have been an effective instrument for achieving a political settlement of conflict," Pankin told the Security Council.

Pankin's remarks come after the United States and the European Union have imposed several rounds of sanctions on Russian individuals and companies to try to persuade President Vladimir Putin to halt any interference in Russian-speaking areas of eastern Ukraine amid the country's escalating crisis.

China, the biggest investor in South Sudan's oil industry, has previously said it would "conscientiously participate" in Security Council discussions, but stopped short of saying whether it would support sanctions for South Sudan. (Reporting by Michelle Nichols; Editing by G Crosse and Mohammad Zargham)