The largest recorded Ebola outbreak in history has led the U.S. Peace Corps to evacuate hundreds of volunteers from three affected West African countries, and a U.S. State Department official on Wednesday said two volunteers were under isolation after having contact with a person who later died of the virus.

Meanwhile, Liberia's president ordered the nation's schools to shut down and most civil servants to stay home as fears deepened over the virus that already has killed more than 670 people in West Africa.

The Peace Corps said it was evacuating 340 volunteers from Liberia as well as neighbouring Guinea and Sierra Leone.

The U.S. State Department official said the two volunteers were not symptomatic and were under observation. The official was not authorized to discuss the cases for attribution, and declined to say where the volunteers were serving or when they were exposed.

Ebola has no vaccine and no specific treatment, with a fatality rate of at least 60 percent.

President Ellen Johnson Sirleaf, who is skipping a summit of African leaders in Washington this week amid the crisis, also called for the closure of markets in an area near the borders with infected countries Guinea and Sierra Leone.

"My fellow Liberians, Ebola is real, Ebola is contagious and Ebola kills," she warned. "Denying that the disease exists is not doing your part, so keep yourselves and your loved ones safe."

Fear and panic over the mounting death toll has prompted some rural communities to accuse foreign aid workers of bringing the deadly virus. Others have kept people with Ebola symptoms at home instead of bringing them to quarantine centres. In anger, one man recently set fire to part of the health ministry building in the Liberian capital, Monrovia, after his teenage brother reportedly died of Ebola.

Sirleaf said that security forces would enforce the new precautions taken a week after an American man of Liberian descent boarded a plane in Monrovia and flew to Nigeria, where authorities said he died of Ebola. The fact that he was able to board a plane and travelled through a major airport transit hub in Togo has heightened fears about Ebola's possible spread in the region.

The airline involved, ASKY, has suspended its flights to both the capitals of Liberia and Sierra Leone, and said passengers departing from Guinea would be carefully screened.

Experts say the risk of travellers contracting Ebola is considered low because it requires direct contact with bodily fluids or secretions such as urine, blood, sweat or saliva. The virus can't be spread like flu through casual contact or breathing in the same air.

Patients are contagious only once the disease has progressed to the point they show symptoms, according to the World Health Organization. The most vulnerable are health care workers and relatives who come in much closer contact with the sick.

Still, Liberia is among the poorest countries in the world, and the outbreak already has taxed the capacity of local health clinics and doctors, prompting concern for those who remain in the country.

Two U.S.-based missionary groups have ordered the evacuation of their non-essential personnel from Liberia after a doctor and a missionary both contracted Ebola.

SIM USA President Bruce Johnson announced Tuesday that his group and Samaritan's Purse decided on the evacuation following an upsurge in the number of Ebola cases in Liberia. A Texas-trained doctor and a missionary from North Carolina have contracted the disease and are in isolation in Liberia.

U.K. officials hold crisis meeting

British officials held a crisis meeting Thursday and warned of a potentially serious threat if the deadly Ebola virus spreads further.

U.K. Foreign Secretary Philip Hammond said there are no known cases in Britain, but that the incurable disease is viewed by the government as a "very serious threat."

He chaired the government's crisis session to discuss possible measures to cope with the disease.

Health officials are being warned to be vigilant for symptoms of the virus. There are concerns it may spread from West Africa, where the current outbreak has already killed hundreds of people.

British public health official Dr. Brian McCloskey said the risk to British travellers and to Britons working in Guinea, Sierra Leone and Liberia is "very low" but that the outbreak isn't under control.

Influenced by media coverage

Tarik Jasarevic, a World Health Organization spokesman, told CBC News Network that the geographic scope of the outbreak makes it particularly difficult to deal with.    

"We need to have enough treatment centres so people who are sick can be treated," he said from Gueckedou, Guinea. "We need to have teams in different locations to follow people who have been in close contact with infected people, and we need to have enough laboratory capacity to be able to test people."
 
Jasarevic said some people in the affected area were influenced by media coverage that highlighted Ebola's lack of cure, which left people wondering why they shouldn't stay to die with their families. "We have to say that the sooner a person gets to the treatment centre, the better it is."

Jasarevic said most people have little to fear from Ebola.

"This disease is not water-borne or air-borne," he said. "So normally, people who are at the highest risk are health workers — especially when they don't know that they dealing with Ebola."

As of July 27, as many as 729 deaths have been blamed on Ebola in Liberia, neighbouring Guinea, Sierra Leone and Nigeria, according to World Health Organization figures. Of that death toll, 485 deaths were confirmed as being linked to Ebola, 205 were likely linked and 39 are suspected of being caused by Ebola.

Liberia accounted for about one-fifth of those deaths. Nigeria had only one death.

Between July 24 and July 27 alone, there were 122 new cases in the four countries and 57 deaths reported.

With files from CBC News and Reuters