UNITED NATIONS — The international aid 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” as the world struggles to contain the disease.

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.

Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said the situation is now the world’s first Ebola epidemic, given how widely it’s spreading in Guinea, Liberia and Sierra Leone.

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.

In Liberia on Tuesday, a missionary organization announced that another American doctor had become infected.

The male obstetrician was not immediately identified by the group Serving In Mission, but he did not work in an Ebola ward, the group said. It did not specify how he contracted the disease, but it can be spread through bodily fluids.

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 received an experimental drug known as ZMapp. The manufacturer says it has run out of supplies of the drug and that 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 U.S. Department of Health and Human Services to accelerate the manufacturing process.

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

On Monday, nurses at a major hospital in the Liberian capital went on strike, according to spokesman Jerald Dennis III. Information Minister Lewis Brown said late Tuesday that the dispute had been resolved, but Dennis said discussions were ongoing.

Meanwhile, the Sierra Leone government said nurses were back at work Tuesday after a strike at a Freetown hospital over the weekend. The government has said it will pay out all accrued hazard pay and double the allowance going forward.

Earlier 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 to halt the spread of the virus. Surrounding countries have closed land borders, and airlines have suspended flights to and from the affected countries. Seaports are losing traffic, restricting food imports to the hardest-hit countries.

Those countries — Guinea, Liberia and Sierra Leone — all rely on grain from abroad to feed their people, according to the U.N. food agency.

“Even prior to the Ebola outbreak, households in some of the affected areas were spending up to 80 percent of their incomes on food,” said Vincent Martin, who is coordinating the food agency’s response to the crisis. “Now these latest price spikes are effectively putting food completely out of their reach.”

Information for this article was contributed by Mike Stobbe, Jonathan Paye-Layleh, Clarence Roy-Macaulay, Marc-Andre Boisvert and Lauran Neergaard of The Associated Press.