AFP/Abuja

The number of confirmed Ebola cases in Nigeria has risen by one to 11, Health Minister Onyebuchi Chukwu said on Thursday.

The figure includes three deaths and eight people who are being treated at a special isolation unit set up in Lagos, a megacity of more than 20 million people, the minister told reporters.

Nigeria has not recorded a case outside Lagos but there were fears that a nurse who was infected in the city may have carried the virus to the key eastern city of Enugu.

The nurse was among those who treated Liberian government employee Patrick Sawyer, who brought Ebola to Lagos on July 20. He died in hospital on July 25.

After contracting the virus, the nurse travelled to Enugu, where she began showing symptoms before being transported back to Lagos for treatment.

Officials on Wednesday said 21 people in Enugu who had contact with the nurse were being monitored.

But Chukwu said Thursday that 15 of those people had been cleared, with only six in Enugu still being watched.

The development is a positive one for health workers in Africa's most populous country, as a spread of the outbreak beyond Lagos would put significant strain on Nigeria's weak healthcare system.

The number of people under surveillance for possible infection in Lagos was reduced from 177 to 169 overnight, Chukwu added.

The virus has a maximum incubation of 21 days, so anyone who does not become symptomatic within three weeks of exposure is thought to be safe.

The worst-ever outbreak of Ebola has killed more than 1,000 people since the start of the year in Guinea, Liberia and Sierra Leone. Nigeria last month became the fourth west African country affected.