KHARTOUM (Reuters) - South Sudan's main party on Monday accused the north of building up troops in the country's highly sensitive Blue Nile border state during a vote count at the climax of troubled national elections.

Blue Nile, on Sudan's north-south border next to Ethiopia, was one of the main battle grounds in the 1983-2005 civil war between north and south Sudan.

It remains a potential flashpoint as the oil-producing country moves through elections towards 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.

A Sudanese police source said they were increasing their presence ahead of the results announcement. "They are executing the security plan that was set out for them ahead of the elections," the source said. "These are ordinary measures."

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 told Reuters after a news conference.

Arman would not say what the SPLM would do, but said his party's leadership would discuss a response.

An SPLM official in Blue Nile, Suleiman Osman, told Reuters more than 2,000 heavily armed police with 18 vehicles mounted with machine guns moved onto the streets of the state capital of Damazin overnight.

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."

HOTLY CONTESTED

Both SPLM and NCP have claimed victory in the hotly contested state ahead of the announcement of official results.

Northern and southern leaders could not agree on the future of Blue Nile and neighbouring South Kordofan when they negotiated the 2005 peace deal.

Both states remain in northern Sudan but the residents were promised "popular consultations" giving them a vague right to voice a desire for more autonomy if they are unhappy with the government in Khartoum.

Sudan's elections were marred by poor preparation and 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 -- Sudan's first open polls in more than two decades -- suggest President Omar Hassan al-Bashir and his party are heading for an overwhelming victory.

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.

Elections officials told Reuters it would likely be some days before presidential results are announced because of technical delays in the reporting systems.

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.