WASHINGTON (Reuters) - Sudan risks "violence on a massive scale" if there is any delay to a planned January referendums that will likely split the oil-rich African nation in two, South Sudan's leader said on Friday.

South Sudan President Salva Kiir said it increasingly appeared that "unity is not an option" following the January 9 vote, which will cap a 2005 peace deal that ended decades of civil war in the country.

"At the moment all signs point to the fact that on January 9, 2011 Southern Sudanese people will vote overwhelmingly for their own independence," Kiir told an audience in Washington.

"There is without question a real risk of a return to violence on a massive scale if the referenda do not go ahead as scheduled," he said.

U.S. President Barack Obama will join other world leaders at a U.N. summit on Sudan next week in a sign of mounting concern that the January vote could reopen a 20-year conflict responsible for 2 million deaths, mostly from hunger and disease.

The United States has intensified its diplomatic engagement with both sides, and this week offered South Sudan and the northern government in Khartoum a new package of incentives to reach a deal, balanced by the threat of new punitive measures including sanctions if progress stalls.

Kiir said his government was working out final details on issues including borders and citizenship, as well as to find a mutually acceptable formula on how North and South Sudan will split the country's oil revenues.

HONORING OIL DEALS

The oil question in particular has emerged as a key sticking point, with most of Sudan's oil production in Abyei and the South but most of the revenues going to the North, which controls the export channels.

"There have worrying signs of foot-dragging by our partners in the North," Kiir said, calling on the United States and the rest of the international community to ensure that all past and future deals between the two sides are honored.

"There are rising calls that the South must make accommodations and compromises if it expects the North to accept its independence," Kiir said, adding that he was troubled by the notion that the South would have to "buy its freedom" by surrendering its oil rights.

All sides concede that time is short to prepare for a credible vote, and that if voter registration does not begin next month it could push the entire process of track.

Kiir, whose ruling Sudan People's Liberation Movement (SPLM) has much riding on the outcome of the plebiscite, said that Western standards should not necessarily apply when judging whether the vote was successful.

"Southern Sudan is not like Switzerland," Kiir said. "It is not realistic to demand perfection."