A Liberian man in his 40s is being tested for the deadly Ebola virus in Nigeria's commercial capital of Lagos, a city of 21 million people, the Lagos State Health Ministry said.
Ebola has killed 632 people across Guinea, Liberia and Sierra Leone since an outbreak began in February, straining a string of weak health systems despite international help.
This would be the first recorded case of one of the world's deadliest diseases in Nigeria, Africa's biggest economy and most populous nation with 170m people.
Nigeria is home to some of the continent's least adequate health infrastructures for a nation of its size, despite access to billions of dollars of oil money as Africa's biggest oil producer.
The special adviser on public health to the Lagos state government, Yewande Adeshina, told a news conference the man had arrived in Lagos from Liberia on Sunday.
"The patient was admitted and detained on suspicion of possible EBV (Ebola) infection, while blood sample collection and testing was initiated," she said in her statement.
Samples had been sent to a World Health Organisation laboratory in Dakar, Senegal, she said, adding that "results are pending."
The Ebola outbreak started in Guinea's remote southeast and has since spread across the region's poorly controlled borders.
Symptoms of the highly infectious disease are diarrhoea, vomiting and internal and external bleeding.