Two alarming new cases of Ebola have emerged in Nigeria, widening the circle of people sickened beyond the immediate

group of caregivers who treated a dying airline passenger in one of Africa's largest cities.

The outbreak also continues to spread elsewhere in West Africa, with 142 more cases recorded, bringing the new total to 2,615 with 1,427 deaths, the World Health Organization said Friday.

Most of the new cases are in Liberia, where the government was delivering donated rice to a slum where 50,000 people have been sealed off from the rest of the capital in an attempt to contain the outbreak.

New treatment centres in Liberia are being overwhelmed by patients that were not previously identified. One centre with 20 beds opened its doors to 70 possibly infected people, likely coming from "shadow-zones" where people fearing authorities won't let doctors enter, the UN health agency said.

"This phenomenon strongly suggests the existence of an invisible caseload of patients who are not being detected by the surveillance system," the agency said. This has "never before been seen in an Ebola outbreak."

A staffer for Doctors Without Borders suits up in protective clothing before entering a high-risk area of the MSF Ebola treatment centre on Thursday near Monrovia, Liberia. The MSF centre has 120 beds and is being expanded, making it the largest Ebola treatment centre in history. ( John Moore/Getty)

The two new cases in Nigeria were infected by their spouses, both medical workers who had direct contact with Liberian-American Patrick Sawyer, who flew into Nigeria from Liberia and Togo and infected 11 others before he died in July. The male and female caregivers also then died of Ebola, Health Minister Onyebuchi Chukwu said Friday.

Nigerian officials initially claimed the risk of exposure to others was minimal because Sawyer was whisked into isolation after arriving at the airport. Lagos state health commissioner Jide Idris later acknowledged that Sawyer was not immediately quarantined.

The two new cases were quarantined two days ago while being tested, Chukwu said. They had previously been under surveillance, meaning they were contacted daily to see if they developed any symptoms and they were advised to stay at home and avoid contact with others, but they were not forcibly restricted. Once they showed signs of the disease, they were brought in to a quarantine unit. People who contract Ebola are not contagious until they show symptoms, say health experts.

Authorities are now trying to identify and monitor everyone they have been in contact with.

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In all, 213 people are now under surveillance in Nigeria, including six people, all "secondary contacts" like the caregivers' spouses, being monitored in the state of Enugu, more than 500 kilometres east of Lagos.

A mobile laboratory capable of diagnosing the disease has been moved there, Chukwu said.

Nigeria's total of confirmed infections is now 12. Five of them have died and five have recovered; the rest are being treated in isolation in Lagos, the commercial capital where Sawyer's flight landed.

The damage has been far greater in Guinea, Sierra Leone and Liberia, each dealing with hundreds of cases. Liberia has been hit hardest, recording 1,082 cases and 624 deaths.

In Liberia, a teenage boy died after being shot by security forces in West Point, a slum that was blockaded this week to stop the spread of Ebola, a Liberia government spokesman said Friday.

Shakie Kamara was hurt in a clash with police and soldiers who sealed off their peninsula from the rest of Monrovia.