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.