A potentially lethal epidemic of cholera and other disease is set to sweep parts of Somalia and Kenya after severe flooding left hundreds of thousands homeless in the two countries, aid workers have warned.

The Somali prime minister, Hassan Ali Khaire, appealed for international humanitarian intervention after two of the largest rivers in the centre of the country – the Shebelle and the Juba – burst their banks, sending floodwaters coursing through riverside towns and villages.

More than 100,000 people were forced to flee Beledweyne, a town in the Shebelle Valley 206 miles north of the capital Mogadishu, over the weekend, local officials said.