Women wait for food at the United Nations Mission in the Republic of South Sudan for the Protection of Civilians site on June 26 in Tomping. Aid agencies warned on Thursday that by August, some areas of South Sudan are likely to slip into famine. Charles Lomodong / Agence France-Presse

"If the conflict in South Sudan continues, and more aid cannot be delivered, then by August it is likely that some localized areas of South Sudan will slip into famine," warned Britain's Disasters Emergency Committee, a coalition of 13 major aid agencies.

Thousands have been killed in the conflict in the world's newest country, while more than 1.5 million have been forced to flee since the war broke out in mid-December. Peace talks are stalled.

The United Nations has around 40 percent of the cash it needs, with a shortfall of over $1 billion.

"There is a very real risk of famine in some areas," DEC chief Saleh Saeed said, warning that "millions of people are facing an extreme food crisis".

Famine implies that at least 20 percent of households face extreme food shortages, that there is acute malnutrition in over 30 percent of people, and that two deaths per 10,000 people every day, according to the UN's definition.

Rains this year are hoped to be around average or slightly below, according to UN experts, with hunger caused by fighting, not extreme climatic conditions.

The UN children's agency, UNICEF, warned in April that nearly 250,000 children in South Sudan will suffer severe acute malnutrition this year if more action is not taken, the Associated Press reported on Thursday.

The DEC coalition, which includes agencies such as Oxfam, Tearfund and Save the Children, "have less than half the money they need to help prevent the growing food crisis in South Sudan turning into a catastrophe," Saeed added.

Fighting between forces of President Salva Kiir and troops loyal to rebel chief Riek Machar has been marked by widespread atrocities.

Kiir and Machar committed themselves last month to a third cease-fire deal, and agreed to forge a transitional government within 60 days, but fighting continues.

"Although humanitarian agencies are making every effort to increase aid deliveries, access remains limited by fighting and the start of the rainy season, which has turned many unpaved roads into rivers of mud," the aid agencies added.