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KHARTOUM (Reuters) - Sudan's former southern rebels said on Saturday they would boycott elections in a key oil-producing state over concerns about fraud, ratcheting up the pressure less than three months ahead of voting.

The former rebel Sudan People's Liberation Movement (SPLM) planned to boycott "all elections" in the central state of Southern Kordofan, the party's candidate for state governor Abdel Al Aziz Adam Al Hilu told reporters in Khartoum.

The April elections across Africa's largest country were promised under a 2005 peace deal that ended more than two decades of north-south civil war.

Southern Kordofan -- the site of oilfields and important civil war battlegrounds on the undefined north-south border -- is set to be one of the most closely watched constituencies.

The former rebels said there had been widespread fraud during November's registration exercise, and that a census used to draw up constituency boundaries had missed out large areas occupied by SPLM supporters.

"This means the elections will not be fair ... We will not take part in these elections unless they repeat the census or redraw the geographical constituencies," Hilu said.

The ruling National Congress Party (NCP) has denied the fraud allegations and also dismissed the boycott announcement.

"This is more about the problems the SPLM is having in gathering support and their internal problems in Southern Kordofan," Ibrahim Ghandour, a senior NCP official, told Reuters, declining to go into further detail.

The SPLM has used boycotts before to push for concessions, walking out of parliament last year over democratic reforms.