KHARTOUM (Reuters) - Three drivers from the U.N. World Food Programme have been killed by bandits in separate attacks over the past week in Sudan's war-stricken western Darfur region and in its south, the agency said on Wednesday.

The attacks highlight the security dangers in Sudan, where the world's biggest humanitarian operation is trying to cope with the crisis in Darfur while aid agencies are trying to help the south rebuild after decades of civil war.

On Monday a driver working for WFP was shot dead and his assistant seriously wounded in South Darfur state. On March 22, two drivers on their way to the oil-rich Abyei state were stabbed to death in south Sudan by six assailants, WFP said.

In Darfur alone, the agency has had 56 trucks and six passenger vehicles hijacked this year and 24 drivers are still missing, the agency said.

"We are extremely shocked and saddened by these incidents. This situation is completely unacceptable," the agency's head in Sudan, Kenro Oshidari, said in a statement.

More than 12,000 humanitarian workers provide aid to around two-thirds of Darfur's population as a five-year-old rebellion drags on. International experts put the death toll at 200,000 although the Khartoum government says it is about 9,000.

WFP said hijackings were rare in south Sudan, where a 2005 north-south peace deal ended Africa's longest civil war.