"Twenty confirmed dead," said Yushau Shuaib of the flooding in the city of Ibadan. "A dam overflowed its boundary ... It's a very serious situation."

The floods resulted from heavy rains that began on Friday. Rubbish and other debris blocking waterways prevented water from properly flowing after the dam overtopped, said Shuaib.

Local media reports spoke of thousands displaced. Shuaib said it was impossible to get a headcount on the displaced for the time being, but that it was "definitely in the thousands."

Bridges reportedly collapsed in the flooding that also left cars submerged. Rescue workers were assessing the situation and seeking to assist those affected.

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.