About two million people died in the 22-year war between south Sudanese rebels and the government in Khartoum, according to the UN, in a conflict fuelled by religion, ethnicity, ideology, resources and oil.

"Since the signature of the peace accords in January 2005, more than two million people have returned to south Sudan," IOM spokesperson Jean Philippe Chauzy told journalists.

He said the pace of return slowed significantly after more than 1.47 million went home during the first two years, with just 160,000 going back in 2009.

There had been signs of a pick up in returns in the first weeks of this year, ahead of the country's general elections in April, he said.

60% of returning families were headed by just one surviving parent, he added.

Tensions remain high in the country between the mainly Muslim north and the underdeveloped south, most of whose inhabitants are Christian or follow traditional beliefs.

About 4.3 million people in the south relied on food aid last year, according to the UN's World Food Programme.

Last week political parties in the south pledged to ensure upcoming polls in April are free and fair in the wake of violent clashes in the region.

They also signed a separate commitment to support the 2005 peace agreement, which sets up a independence referendum for the south due in 2011.