Nigerian President Goodluck Jonathan has visited some of the hundreds of thousands of people made homeless by the country's worst flooding in at least 50 years, calling it a "national disaster".

At least 140 people have been killed, hundreds of thousands uprooted and tens of thousands of hectares of farmland have been submerged since the start of July, raising concerns about food security, the National Emergency Management Agency (NEMA) said on Friday.

Vast stretches of Nigeria has been submerged by floods in the past few weeks, as major rivers like the Niger, the continent's third longest, burst their banks.

 An estimated 623,900 people had been displaced and 152,575 hectares of farmland destroyed so far [Reuters]

In Kogi, a northern state that has been the worst affected and which Jonathan visited on Thursday, NEMA state coordinator Ishaya Chonoko said 623,900 people had been displaced and 152,575 hectares of farmland destroyed so far.

"We are very sad over these flood incidences in the country.

It is a national disaster," Jonathan said with a sombre expression, after casting an eye over the makeshift displacement camp huddling 738 people together in Dankolo primary school.

"We are thinking of how to settle you all back to your places after the floods. Government is doing everything possible to cushion the effects on you ... it will soon be over."

Nigeria, which gets heavy tropical rains from May to September, usually suffers from seasonal flash floods but almost never on this scale.

Floods have also devastated the Niger Delta, home to Africa's biggest energy industry, where the Niger river fans into creeks before emptying itself into the Atlantic.

There has been no reported impact on crude oil production, but a cocoa industry body said last month that cocoa output would fall far short of a 300,000 tonne target for last season.

Source: Agencies