Floods triggered by a heavy downpour this week killed at least 20 people in Nigeria\'s largest city of Lagos, emergency services officials said on Tuesday. \"Not less than 20 people including children were killed in the Lagos flooding on Sunday,\" National Emergency Management Agency (NEMA) spokesman Yushau Shuaib said. The flooding described by the agency as \"the most devastating so far this year in Nigeria\" hit one of Africa\'s largest cities on Sunday and Monday, turning roads into rivers and forcing many people out of their homes. Lagos, a city of 15 million people, is overcrowded, with many residents living in hapharzardly built slums. Tunde Adebiyi, a NEMA coordinator in Lagos, said he had a partial casualty figure of 18 and was still receiving more statistics. At least four children were among the dead, he said, adding that \"apart from these, we also heard of three other children elsewhere, but we are still to confirm.\" The agency also confirmed the death of nine people in the northern city of Katsina in rain-induced floods also on Sunday. Flooding occurs each rainy season in Nigeria, though emergency officials have warned of particularly intense rains this year. The rainy season typically runs from around April to September.