Cholera cases in war-torn Sudan increased by nearly 40% in less than two weeks, according to the latest figures, alarming the U.N. health officials, who long warned about the outbreak and the lack of proper response since it was reported in July.

Sudan’s cholera outbreak has killed 545 people and infected more than 18,000, the health ministry said on Tuesday, as the country struggles to contain the spread of the disease. The outbreak began in August following heavy rains and flooding,

The United Nations is concerned over the rapid spread of cholera in Sudan, where more than 600 people have died from the disease.