Conflicts, floods and failed rains caused by El Nino have sparked a dramatic rise in the number of people going hungry in east Africa, especially in drought-hit Ethiopia, the United Nations said Thursday.

El Nino, a global weather pattern that periodically wreaks havoc, is expected to last until early 2016.

"Due to El Nino, food insecurity is forecast to worsen over the coming months, especially in Ethiopia," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a report.

"By the beginning of 2016 the number of people who are food insecure and in need of humanitarian assistance is expected to increase to 32.1 million," the report said, adding that "up to two million people could be affected by flooding."

Since May, numbers in need have already risen by 6.7 million, from 18.5 million to 25.3 million people, the report added.

While some countries -- including Sudan, Eritrea, Ethiopia and Djibouti -- could see drier conditions, other countries including Kenya, Somalia and Uganda are at risk of floods.

Over 90,000 people in war-torn southern Somalia have already been hit by weeks of severe flooding, almost half of them forced from their homes, the UN said.

In neighbouring Ethiopia, El Nino has "wreaked havoc" on summer rains needed to grow crops, with around 8.2 million people needing food aid, nearly double the number in need six months ago.

"This comes on the heels of failed spring rains, and has driven food insecurity, malnutrition and water shortages," the UN said, expressing fears the numbers affected "will continue to grow."

El Nino is triggered by a warming in sea surface temperatures in the Pacific Ocean. It can cause unusually heavy rains in some parts of the world and drought elsewhere.

The UN children's agency, UNICEF, warns that 350,000 children in Ethiopia are "expected to require treatment for severe acute malnutrition by the end of 2015," with hunger, disease and lack of water likely to kill many.

"El Nino can lead to significant increases in diseases such as malaria, dengue fever, diarrhoea and cholera - which are major killers of children," UNICEF said.

A civil war across the Gulf of Aden in Yemen that has sparked an outflow of refugees to Somalia and Djibouti, as well as a war in South Sudan and violence in Burundi have all compounded the situation, causing refugee numbers to rise.

In South Sudan, where a nearly two-year long civil war rumbles on, some 40,000 people are already starving, with tens of thousands more on the brink of famine, the UN has said.