UNITED NATIONS, March 3 (Reuters) - At least 50,000 people have been killed in South Sudan's two-year civil war, a senior United Nations official said on Wednesday, a five-fold increase in the death toll given by humanitarian agencies in the early months of the conflict.

A political dispute between President Salva Kiir and his former deputy, Riek Machar, sparked the war in December 2013, which has reopened ethnic fault lines between Kiir's Dinka and Machar's Nuer people.

"Fifty thousand killed, maybe more, 2.2 million refugees and displaced, famine coming and looming in just a few months," the senior official, who spoke on condition of anonymity, told a small group of reporters. He added that he saw little prospect of implementing an August peace deal.

The United Nations said last month that South Sudan's warring parties are killing, abducting and displacing civilians and destroying property despite conciliatory rhetoric by Kiir and Machar.

After months of ineffective negotiations and failed ceasefires, both sides agreed in January to share positions in a transitional government, and last month Kiir reappointed Machar to his former post as vice president.

"Where are we on the implementation of the peace agreement? Nowhere," the senior U.N. official said. "We see violence spreading along ethnic lines in other parts of South Sudan which had been spared so far."

A U.N. panel that monitors the conflict in South Sudan for the Security Council stated in January that Kiir and Machar are still completely in charge of their forces and are therefore directly to blame for killing civilians.

U.N. peacekeepers are sheltering nearly 200,000 people at six protection sites in South Sudan.

Angola said last week it had proposed that the U.N. Security Council impose an arms embargo on South Sudan, but veto-power Russia has said it was opposed to such a move as it did not believe it would be helpful.

Last week, U.S. Secretary of State John Kerry said Kiir and Machar would face individual sanctions if they did not deliver on the peace deal, warning of a "critical moment for South Sudan's survival."

The conflict in South Sudan, whose 2011 secession from Sudan had long enjoyed the support of the United States, has torn apart the world's youngest country.

(Reporting by Michelle Nichols and Louis Charbonneau; Editing by Matthew Lewis)