Blue Nile was one of the main battlegrounds in the 1983- 2005 civil war between north and south Sudan and remains a potential flashpoint as the oil-producing country moves through elections toward a 2011 referendum on southern independence.

The elections were set up under the 2005 peace deal that ended that conflict and were supposed to help bring the country back to democracy.

North Sudan's dominant National Congress Party (NCP) dismissed the accusations over troop movements in Blue Nile as "completely false" and there was no one immediately available to comment from Sudan's army.

The southern Sudan People's Liberation Movement (SPLM), a former rebel group, said it had information the north had been building up troops in the region over the past two days and said it would not accept any attempt to influence the results of a hotly-contested race for state governor.

"They are building up forces there ... They are trying to rig the elections ... It is unacceptable. It is a red line and we will not accept it," SPLM presidential candidate Yasir Arman said after a news conference.

Arman would not say what the SPLM would do if that line was crossed but said the party leadership would discuss a response.

"This is completely false,” senior NCP member Rabie Abdelati said. "The early figures show that the NCP will get the victory in Blue Nile. Yasir Arman knows this and this is why he is making these falsehoods." Both the SPLM and the NCP have already claimed victory in the highly contested state, on Sudan's north-south border next to Ethiopia, ahead of any official announcement of results.

SPLM gubernatorial candidate Malik Agar said early results showed he had a majority of more than 13,000. Abdelati said the data showed a 6,000 majority for the NCP's Farah Agar.

Northern and southern leaders could not agree on the future of Blue Nile and neighboring South Kordofan when they negotiated the 2005 peace deal. The states remain in northern Sudan but the populations were promised vague "popular consultations" giving them the right to voice a desire for more autonomy from Khartoum if they are unhappy with the government.

Sudan's elections were marred by poor preparation and other suspected irregularities, the United States, Britain and Norway said on Monday, calling on Sudanese officials to take further steps to fully implement the country's 2005 peace accord.

Early results from the election — Sudan's first in more than two decades — suggest President Omar Bashir and his party are heading for a strong win.

Bashir, wanted by the International Criminal Court to face charges of war crimes in Darfur, scored majorities of up to 90 percent in a sample of results from northern Sudan reported by state media.

European Union and Carter Center observers have said the elections did not meet international standards, but stopped short of echoing opposition charges of vote-rigging. The SPLM and other parties boycotted large parts of the voting.