Heavy rains that caused a dam to overflow in southwestern Nigeria and led to houses being submerged has killed 20 people and displaced thousands, an emergency management agency spokesman said on Sunday.

"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 drainage 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 collapsed in the flooding that also left cars submerged. Rescue workers were assessing the situation and seeking to assist those affected, with the rains having halted Saturday morning.

The main newspaper based in Ibadan published photos of corpses left on the ground, including children, and victims making their way through waist-deep water.

"Lots of bridges have fallen. It is difficult for people to move around," Ibadan resident Bode Olaoluwa, 42, said by phone. "A lot of families have lost children."

He said a friend had informed him of a man who had died in his house with his three children.

The water was gradually receding, but major challenges remained, he said.

"People have to go long distances to go from one place to the other because of the collapsed bridges," said Olaoluwa.

Ibadan is located some 150 kilometres (90 miles) north of Lagos, Nigeria's economic capital and its largest city.

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.

Last month, floods triggered by a heavy downpour killed at least 20 people in Lagos, while 24 died in June when unusually heavy rains inundated a neighbourhood in Nigeria's largest northern city of Kano.

The largest cities in Nigeria, Africa's most populous nation, are overcrowded, with many residents living in haphazardly constructed slums. Drainage systems are also often poorly maintained and contribute to the problem of floods.

Nigeria experienced severe flooding last year that affected around half a million people in two-thirds of its 36 states

The neighbouring nation of Benin was also hard hit by flooding in 2010, with 55,000 homes destroyed and at least 680,000 people affected.

More than 300 people were killed in the 2010 rainy season in western and central Africa.