DAKAR, Senegal -- An ill doctor in southern Nigeria exposed dozens of people to the Ebola virus by continuing to treat patients before his death, the World Health Organization warned Wednesday as it announced the toll across West Africa had surged above 1,900 fatalities.

Officials in Nigeria had believed that Ebola was largely contained within Africa's most populous country after a sick traveller from Liberia brought the disease to Lagos. However, a man who had had contact with the ill visitor later evaded his surveillance and travelled to the oil hub of Port Harcourt where he triggered a second cluster of cases.

A Port Harcourt doctor and another patient there are now dead, and the doctor's widow and sister are sick with Ebola. About 60 other people are under surveillance after having "high-risk" or "very high-risk" contact with the infected doctor, WHO said. More than 140 others are also being monitored.

"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," WHO warned.

Nigeria's health minister has said there is no reason for people to panic in Port Harcourt.

The U.N. health agency, though, said it feared civil unrest and public fear of Ebola could further the crisis, saying "military escorts are needed for movements into the isolation and treatment centre."

Nigeria's Ebola toll so far has been limited in comparison to Liberia, Sierra Leone and Guinea where hundreds have died in each country. Nigerian authorities say five people have died in Lagos, and the doctor in Port Harcourt and the other fatality there bring the national toll to seven.

The man who infected the Port Harcourt doctor was later found after a four-day manhunt and is recovering.

WHO said Wednesday that the physician continued to see patients after the onset of Ebola symptoms and even operated on two people. Ebola is transmitted through direct contact with bodily fluids, and health authorities say patients are only contagious once they show symptoms.

"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," WHO said in a statement.

"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. During his six-day day period of hospitalization, he was attended by the majority of the hospital's health care staff."

The announcement from WHO did not specify whether the health care staff wore gloves or other protective gear when treating him.

Getting protective gear to health workers in the affected areas and ensuring that they receive hazard pay are top priorities for combatting the crisis, said Dr. David Nabarro, who is co-ordinating the U.N. response to the outbreak.

Doctors and nurses have been especially vulnerable to Ebola because they work closely with Ebola patients, whose bodily fluids spread the virus. Dr. Rick Sacra, 51, from the Boston area, is currently infected in Liberia, his missionary organization said Wednesday.

Two other Americans already have been evacuated back to the United States and have recovered from Ebola. Both received an experimental drug known as ZMapp. The company has said that all of its doses are now exhausted, and it will be months before more can be made.

It is also still not clear if the drug is effective, since human trials have not yet been carried out. Two of the people who received ZMapp died, while five others survived.