MONROVIA, Liberia » 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 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 neighboring Guinea and Sierra Leone.

The 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 centers. 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 traveled 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 travelers 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.

Associated Press writers Krista Larson in Dakar, Senegal and Michelle Salcedo in Washington contributed to this report.