Nigerian health authorities on Monday said they were battling a suspected outbreak of yellow fever in two southern areas that local media said might have left dozens dead.
The country's disease control agency said it was supporting authorities in Enugu and Delta states after "reports of a sudden spike in cases and deaths in some communities in both states".
Nigeria's Centre for Disease Control (NCDC) did not give a figure for the dead.
But local health officials quoted in the media said more than 72 people were feared to have died from the disease in the two regions.
There is no specific treatment for yellow fever, which is largely transmitted in urban settings by mosquitos.
The percentage of people immunised against yellow fever remains low in many parts of Africa, even though the vaccine is nearly 100 percent effective and relatively cheap.
The outbreak comes after health experts warned that the coronavirus pandemic could cause some diseases to spike because vaccination programmes were interrupted.
In October, the World Health Organization announced that Nigeria was resuming yellow fever vaccines after stepping up safety measures due to Covid-19, and aimed to reach 30 million people.
Africa's most populous nation - like the rest of the continent - has so far been spared the brunt of the coronavirus pandemic but authorities are wary of a fresh wave.
So far the virus has infected more than 64 000 people and killed 1 154 in the country.