The World Food Program says it will soon begin air-dropping food aid to tens of thousands of refugees in South Sudan.

The U.N. food agency said Thursday the situation has grown dire for more than 100,000 people in Upper Nile State, including 35,000 recent arrivals.

Spokeswoman Rene McGuffin says the start of the rainy season has made it hard to deliver food by ground, and is spreading sickness among those fleeing their homes.

“These refugees are arriving in weakened condition, and many of whom are battling illness, particularly with the rains.”

Most of the refugees have crossed from Sudan to escape violence in the states of Blue Nile and Southern Kordofan. Sudan's armed forces have been fighting rebels in both states since last year.

Health workers report an alarming number of children among the newest refugees are weak, sick and malnourished.

WFP says it will begin air dropping 2,000 metric tons of food to Maban County beginning in mid-August. It also plans to drop about 3,000 metric tons of food to the Yida refugee camp.