Most of Sudan's political opposition parties threatened on Thursday to boycott upcoming elections, following months of concerns raised in the south over insecurity and poor preparation for the polls, according to news reports.

The largest opposition party, the Sudanese People's Liberation Movement, said on Wednesday it was withdrawing its presidential candidate, but it would contest parliamentary and municipal elections.

President Omar al-Bashir, who is the subject of an arrest warrant issued by the International Criminal Court at the Hague for war crimes committed in the Darfur region of western Sudan, now faces only one major challenger in the April 11-13 polls. They would be the first held in Sudan in more than two decades.

In recent months, southern opposition parties have complained that the government had done nothing to prepare for or facilitate voting in southern Sudan, such as providing ballot materials, readying polling places and working to ensure access across the region's vast, roadless spaces. Additionally, reports say hunger and desperation have been growing.

A peace accord in 2005 ended more than 20 years of civil war between Sudan's north and south. The elections are aimed at sealing the peace.