KHARTOUM - South Sudan's main party said on Tuesday it would boycott elections in most states in northern Sudan, in a further blow to the poll already hit by accusations of fraud less than a week ahead of voting. "We announce the SPLM (Sudan People's Liberation Movement) boycott of all the elections in the north on all levels ... in 13 states of the north," party secretary general Pagan Amum told reporters, saying Sudan's incumbent president Omar Hassan al- Bashir had rigged the vote. Sudan's presidential, legislative and gubernatorial elections, due to start on Sunday, are the centrepiece of a troubled 2005 peace deal signed between the SPLM and Bashir's northern National Congress Party (NCP) ending more than two decades of civil war. The SPLM's move will be seen as a rebuke to Bashir and will stoke tension as both sides prepare for an even more sensitive referendum on the independence of Sudan's oil-producing south, also promised under the peace deal, in January 2011. Amum told reporters the party was making the decision in protest at widespread irregularities in the build-up to the vote -- the SPLM and opposition parties have accused the NCP of clamping down on campaigning and rigging voter registration and other preparations. He said the decision excluded the border states of Blue Nile and South Kordofan, adding the SPLM would continue to run at all levels in south Sudan. Tuesday's announcement marked an escalation in SPLM protests at irregularities in the elections. Last week the SPLM said it was pulling out just of presidential elections and all votes in the strife-torn Darfur region, leaving opposition groups in disarray over whether to follow suit. Analysts say Bashir is hoping to show he can win a competitive election to legitimise his rule and fend off his indictment by the International Criminal Court to face charges of war crimes in Darfur.