Nigerian authorities on Monday confirmed a second case of Ebola in Africa's most populous country, an alarming setback as officials across the region battle to stop the spread of a disease that has killed more than 700 people.

Also Monday, health authorities in Liberia ordered that all those who die from Ebola be cremated after communities opposed having the bodies buried nearby. Over the weekend, health authorities in the West African country encountered resistance while trying to bury 22 bodies in Johnsonville, outside the capital Monrovia. Military police helped restore order.

In Nigeria, Health Minister Onyebuchi Chukwu said test samples are pending for three other people who had shown symptoms of Ebola, and that authorities are trying to trace and quarantine others. The confirmed second case in Nigeria is a doctor who had helped treat Patrick Sawyer, the Liberian-American man who died July 25 days after arriving in Nigeria from Liberia.

"Three others who participated in that treatment who are currently symptomatic have had their samples taken and hopefully by the end of today we should have the results of their own test," Chukwu said.

The emergence of a second case raises serious concerns about the infection control practices in Nigeria, and also raise the specter that more cases could emerge. It can take up to 21 days after exposure to the virus for symptoms to appear. They include fever, sore throat, muscle pains and headaches. Often nausea, vomiting and diarrhea follow, along with severe internal and external bleeding in advanced stages of the disease.

Doctors and other health workers on the front lines of the Ebola crisis have been among the most vulnerable to infection as they are in direct physical contact with patients. The disease is not airborne, and only transmitted through contact with bodily fluids such as saliva, blood, vomit, sweat or feces.

Sawyer, who was traveling to Nigeria on business, became ill while aboard a flight and Nigerian authorities immediately took him into isolation upon arrival in Lagos. They did not quarantine his fellow passengers, and have insisted that the risk of additional cases was minimal.

Nigerian authorities said a total of 70 people are under surveillance and that they hoped to have eight people in quarantine by the end of Monday in an isolation ward in Lagos. The emergence there is particularly worrisome because Lagos is the largest city in Africa with some 21 million people.

Health officials rely on "contact tracing" — locating anyone who may have been exposed, and then anyone who may have come into contact with that person. That may prove impossible, given that Sawyer's fellow passengers journeyed on to dozens of other cities and the health workers who treated them may have exposed family members. Nigeria is the fourth country to report Ebola cases and at least 728 other people have died in Guinea, Sierra Leone and Liberia.