UNITED NATIONS — The international group Doctor Without Borders warned Tuesday that the world is losing the battle against Ebola and lamented that treatment centers in West Africa have been “reduced to places where people go to die alone.”

In separate remarks after a United Nations meeting on the crisis, the World Health Organization chief said everyone involved had underestimated the outbreak, which has now killed more than 1,500 people in Guinea, Liberia, Sierra Leone and Nigeria. U.N. officials implored governments worldwide to send medical workers and material contributions.

Meanwhile in Liberia, a missionary organization announced that another American doctor has become infected.

Doctors Without Borders, which has treated more than 1,000 Ebola patients in West Africa since March, is completely overwhelmed by the disease, said Joanne Liu, the organization’s president. She called on other countries to contribute civilian and military medical personnel familiar with biological disasters.

“Six months into the worst Ebola epidemic in history, the world is losing the battle to contain it,” Liu said at a U.N. forum on the outbreak. “Ebola treatment centers are reduced to places where people go to die alone, where little more than palliative care is offered.”

In Sierra Leone, she said, infectious bodies are rotting in the streets. Liberia had to build a new crematorium instead of new Ebola care centers.

At the U.N. meeting, WHO Director Margaret Chan thanked countries that have helped but said: “We need more from you. And we also need those countries that have not come on board.”

Later at a news conference, she warned that the outbreak will get worse before it gets better.

President Obama urged West Africans on Tuesday to wear gloves and masks when caring for Ebola patients or burying anyone who died of the disease. He discouraged the burial practice of directly touching the body of Ebola victims, which is one way the disease has been spreading.

“You can respect your traditions and honor your loved ones without risking the lives of the living,” Obama said in the brief video message.

Last month, two Americans, including one from the same missionary group, were evacuated to the United States for treatment after contracting Ebola in Liberia. The two recovered after receiving an experimental drug known as ZMapp. The manufacturer says it has run out of supplies of the drug and it will take months to produce more.

U.S. health officials on Tuesday announced a $24.9 million, 18-month contract with Mapp Biopharmaceutical Inc. to speed development of ZMapp. As part of the project, Mapp is to make a small amount of the drug for early-stage safety testing, while working with the Department of Health and Human Services to accelerate the manufacturing process.

The outbreak has taken a particularly high toll on health care workers, and nurses in Liberia and Sierra Leone have repeatedly gone on strike to demand hazard pay and better protective gear.

Also Tuesday, the U.N. Food and Agriculture Organization warned that food in countries hit by Ebola is becoming more expensive and will become scarcer because some farmers can’t reach their fields.

Authorities have cordoned off entire towns in an effort to halt the virus’ spread.