The agency in charge of two dams in northern Nigeria denied on Sunday that the opening of gates at the barriers had caused major flooding that officials say has displaced some two million people.

The governor of Jigawa state has blamed opened flood gates for the disaster that has destroyed huge swathes of farmland and left residents taking shelter in schools.

State officials say the dams were opened last month, swamping areas along the Hadejia River and adding to flood misery that had already washed away entire villages across other parts of Nigeria's north this rainy season.

"This is not the issue at all," said Salisu B. Hamzat, spokesman for the Hadejia-Jama'are River Basin Development Authority, which oversees the two dams Jigawa state officials blame for the flooding.

"This year we have had heavy rainfall almost everywhere in the country. This is actually what caused the flooding."

Hamzat said the dams, located in Kano state, which borders Jigawa, are never manually opened and simply empty automatically into a spillway once the reservoir fills.

The larger of the two dams, the Tiga dam, has not spilled over this year, according to Hamzat.

He said the smaller dam, the Challawa, has spilled over, but the amount involved would not be nearly enough to cause the flooding that has occurred in Jigawa.

Two people were killed in the initial stages of the flooding in Jigawa state, a Red Cross official has said.

Many of the villages in the flooded areas consisted of mud-and-thatch huts and were populated by fishermen who work the river and farmers growing maize, millet and rice.

Local government officials have heavily criticised the dam agency, saying similar incidents have occurred in the past and they have not changed their approach. They called for the river to be dredged.

About 90,000 hectares (222,400 acres) of farmland have been washed away, the state says.

A range of states in the north of the vast country, Africa's most populous nation, have seen major flooding this year.

In Sokoto state in Nigeria's northwestern corner, local chiefs told workers from aid organisations that around 40 people were killed when a dam burst earlier this month, but the number could not be confirmed.

On Sunday, Doctors Without Borders aid workers set out in boats in Sokoto to access flooded villages.

They have estimated around 40,000 people remain displaced in northern Sokoto, but limited access to the area has made it difficult to come up with an accurate number.

An official with the organisation said by phone on Sunday that a major bridge across the Rima River had collapsed and local residents were using wooden boats to traverse the waterway and flooded roads.

"It appears that quite a lot of this area is still flooded," Gautam Chatterjee said as he traveled the river by boat.

Water had begun to subside in many areas, but cholera and malaria remained risks, he said. Military boats were also out in the area.

Residents were in need of clean water and the organisation was working to help provide it, said Chatterjee.

Nigerian President Goodluck Jonathan, who is running in elections set for early next year, toured Sokoto's flooded areas last week and pledged help.