A week-long referendum in Sudan that will determine if the country divides has seen a high voter turnout, even as reported clashes are said to have killed dozens of people along the country's north-south border in a region not participating in the referendum, U.S. officials said on Tuesday.

 Thus far we are pleased with the high level of turnout and the cooperation of officials in both north and south Sudan, Assistant Secretary for African Affairs Johnnie Carson told reporters on Tuesday.

The process has been peaceful, with only a handful of reported disturbances in Abyei and north of the 1956 north-south border, he said.

The vote, which is due to end Saturday, could create two countries split along religious lines. Sunni Muslims comprise 70 percent of the population and live mostly in the North. Of the other 30 percent of the population, mostly residing in the south, 25 percent hold indigenous beliefs and 5 percent are Christian, according to the CIA World Factbook.

Reports on Tuesday citing southern officials say ten southern Sudanese were killed along the north-south border in an ambush on their motorcade as they traveled to the south  to vote in the referendum. Southern officials blamed armed nomads of the northern Misserya tribe.

A spokesman from the tribe called the assertion a lie, according to a published report.

U.S. Ambassador Princeton Lyman, who has served as a negotiator in Sudan, said the reported clashes in the Abyei district created a worrisome situation. He noted that the district was not taking part in the referendum because there was no agreement on voter eligibility.

He called it an extremely important, sensitive issue to be resolved in the future. The long term resolution in Abyei is political, he said.

He said American officials were located in five of the 10 southern states as observers, with officers traveling to other states to observe the voting process. The referendum is the last phase of a Comprehensive Peace Agreement reached in 2005.

Beyond the current vote, there are big issues out there to be resolved, Lyman said.

The parties were either not ready or not in position to address them before the referendum, he said.

He said that among the issues to be dealt with are the management of the oil sector, finalization of some disputed border areas, questions of citizenship, banking and currency arrangements, security, international legal operations and debt.