Sudan started to disarm 180,000 fighters from the war between its north and south on Tuesday, setting into motion a demobilisation plan at the heart of the peace deal that ended Africa's longest civil war.

At a ceremony marking the start of the U.N.-backed campaign, 15 soldiers from north and south Sudan handed over their weapons in exchange for cash and food tokens.

But the funding of the long-delayed programme, designed to ease tens of thousands of soldiers and militia fighters back into civilian life, is still a problem.

So far Japan and France have donated just over $21 million of the $385 million needed to fund it over the next four years.

"I hope that other donors will join their ranks soon," said Ameerah Haq, the U.N.'s humanitarian coordinator, in a speech at the ceremony in Damazin in Sudan's remote Blue Nile state.

Both sides agreed to reduce their troop numbers after the 2005 peace deal, which called for national elections to be held in 2009, and a referendum on southern secession in 2011.

The Disarmament, Demobilisation and Reintegration programme is the largest ongoing programme of its kind, Haq said.

Large parts of Sudan are still awash with guns, many of them left over from the civil war which killed an estimated two million people and lasted for 21 years.

The presence of weapons in towns and villages has exacerbated old tribal conflicts. The resulting insecurity has plagued Sudan's oil-rich south, where regular attacks by gunmen have hindered aid operations and deterred investors.

Southern officials have raised fears that many former soldiers could return to violence without jobs or funding.