The World Health Organization warned that an Ebola outbreak in Nigeria's oil producing hub of Port Harcourt could spread wider and faster than in the financial capital, Lagos.
The UN health body said the arrival of the virus in Port Harcourt, which is 435 kilometres (270 miles) east of Lagos, showed "multiple high-risk opportunities for transmission of the virus to others".
The haemorrhagic fever, which has hit five countries in West Africa and caused nearly 2,000 deaths this year, first arrived in Nigeria when a Liberian finance ministry official died in Lagos on July 25.
He was taken from the city's airport to a private hospital by two officials from the West African regional bloc ECOWAS.
One of the officials later died of the disease but the other evaded detection to travel to Port Harcourt, where he fell ill and was treated in secret at a city hotel room by medical doctor Ike Enemuo from August 1-3.
The ECOWAS official recovered but the WHO said Enemuo continued to treat patients at his private clinic and operated on at least two people, despite showing symptoms from August 11 of Ebola -- of which he later died.
"On 13 August, his symptoms worsened; he stayed at home and was hospitalized on 16 August," the WHO said in an emailed statement.
"Prior to hospitalization, the physician had numerous contacts with the community, as relatives and friends visited his home to celebrate the birth of a baby."
"Once hospitalized, he again had numerous contacts with the community, as members of his church visited to perform a healing ritual said to involve the laying on of hands."
Over the six days he spent in hospital "the majority" of healthcare staff treated him while two doctors performed an abdominal scan at an ultrasound clinic the day before his death, the statement added.
"Given these multiple high-risk exposure opportunities, the outbreak of Ebola virus disease in Port Harcourt has the potential to grow larger and spread faster than the one in Lagos," it added.
A total of 255 people were currently under surveillance in Port Harcourt for signs of Ebola, Nigeria's Health Minister Onyebuchi Chukwu said on Wednesday.
An elderly woman who was a patient at the hospital where Enemuo was treated died from the disease, taking the number of victims in Nigeria to seven out of 18 confirmed cases.
Enemuo's wife -- who is also a doctor -- was in an isolation unit in Lagos, while his sister is under quarantine in Port Harcourt.