The United Nations' humanitarian coordinator in South Sudan says a full-blown humanitarian disaster is set to hit the world's newest country unless $1.27 billion is raised to ramp up vital relief before the approaching rainy season.

Toby Lanzer said Tuesday that more than 900,000 people have been displaced in nearly two months of warfare and that 3.2 million people are in critical need. He said the aid is needed by June, before remote villages are cut off from road access by seasonal rains.

Lanzer noted that South Sudan's recent violence has destroyed many town markets. He said nearly 5,000 tons of food and non-food aid has been stolen during the lawlessness.

Lanzer predicted difficulty in raising the needed money because of crises in Syria, Philippines and the Central Africa Republic.