Almost everyone on board a boat died when it capsized in a flood-swollen river in Nigeria's Anambra state, President Muhammadu Buhari said on Sunday.

An estimated 85 people were taking the overloaded boat on Friday when flooding of the Niger River caused it to tip over.

"The boat reportedly carrying 85 persons capsized following rising floods in Ogbaru area of the state, with emergency services confirming a death toll of 76," Buhari's office quoted him as saying.

He directed the emergency services to provide relief to the victims.

"I pray for the repose of the souls of the deceased and for everyone's safety, as well as the well-being of the family members of the victims of this tragic accident," he added.

Earlier on Sunday, the emergency services said the rising water level was hampering rescue efforts.

"The water level is very high and too risky for a smooth search and rescue operation," Thickman Tanimu, the southeast coordinator of the National Emergency Management Agency (NEMA), told Agence France-Presse (AFP).

He said the flooding was the worst the country has seen in years, with the water level around one-tenth higher than a decade ago.

NEMA requested the Nigerian Air Force provide helicopters for the rescue operation.

Anambra state governor Charles Soludo urged residents of flood-ravaged areas to relocate while adding that the government would provide relief to those affected by the disaster.

"This development is still a shock to the government and good people of Anambra State. I sympathize with the families of people involved," Soludo said in a statement.

Boat accidents occur regularly in Nigeria because of overloading, speeding, poor maintenance and disregarding navigation rules.

Since the start of the rainy season, many regions of the West African nation with more than 200 million people have been devastated by floods.

More than 300 people have been killed and at least 100,000 made homeless, according to the emergency services.

The continuous rain has washed away farmland and crops, sparking fears of food shortages, famine and hunger in a country already struggling with the impact of the COVID-19 pandemic and the war in Ukraine.