The World Food Program says it will soon begin air-dropping food to tens of thousands of refugees who have fled into South Sudan in recent weeks.

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

Most 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.

The sharp increase in refugees comes at the start of South Sudan's rainy season, which has made it difficult for aid groups to deliver lifesaving food by ground.

The World Food Program 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.