A U.S. physician working in Liberia has tested positive for Ebola, the missionary organization SIM USA announced Tuesday.

The unnamed missionary doctor was treating obstetrics patients at the organization's ELWA hospital in Monrovia, Liberia, and was not working with Ebola patients in the facility's isolation unit, which is separate from the main hospital, according to a news release from the organization. He isolated himself immediately upon developing symptoms and has been transferred to the Ebola isolation unit.

The doctor is the fourth American to be diagnosed with the hemorrhagic disease that has killed more than half the people who have become infected during the current outbreak in West Africa. Another physician, Kent Brantly, and a missionary volunteer, Nancy Writebol, who also were working at ELWA hospital, were treated with an experimental medication and brought back to Emory University Hospital in Atlanta, where both recovered.

A Liberian American, Patrick Sawyer, fell ill upon his arrival in Nigeria and died of the disease.

SIM USA will hold a news conference Wednesday to provide more details.

Separately, officials who are battling the outbreak in five African countries warned Tuesday morning that the situation is reaching a critical stage.

“Six months into the worst Ebola epidemic in history, the world is losing the battle to contain it,” Joanne Liu, the international director of Doctors Without Borders, said in a briefing before the United Nations. “Leaders are failing to come to grips with this transnational threat.”

Liu offered detailed examples of how dire the situation has grown in parts of West Africa. She described overwhelmed isolation centers, riots breaking out over controversial quarantines, infected bodies lying in the streets, medical workers dying in shocking numbers, entire health systems crumbling, and Ebola wards with such scant resources that they are little more than where “people go to die alone.”

“We are in uncharted waters,” she said. “[Doctors Without Borders] has been ringing alarm bells for months, but the response has been too late, too little.”

Like other officials Tuesday, Liu said the outbreak could be contained and order restored in West Africa, but only if the international community acts quickly and cooperatively.

“Only by battling the epidemic at its roots can we stem it,” Liu told U.N. leaders. “It is your historic responsibility to act. We cannot cut off the affected countries and hope this epidemic will simply burn out. To put out this fire, we must run into the burning building.”

Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, offered another dire assessment of the situation in a news conference in Atlanta Tuesday after he returned from a visit to Liberia, Sierra Leone and Guinea.

“There is a window of opportunity to tamp this down, but that window is closing. We need action to scale up, and we need to scale up to massive levels….I cannot overstate the need for an urgent response,” he said. On Thursday, the World Health Organization offered a six-month, $490 million plan to contain the outbreak that it said would require thousands of personnel.

The speed of this outbreak's spread makes it "really the first epidemic of Ebola the world has ever known," Frieden said. Over the next few weeks, the situation is likely to get worse, he said, and health officials probably will see a significant increase in the number of cases. There is widespread transmission in Liberia and Sierra Leone. Guinea until now has had more success controlling the outbreak. But Guinea has seen an increase in the number of cases there in the last couple of weeks, he said. The spike has been in one community where many have resisted preventive measures. Some people believed that spraying bleach, a measure taken against the virus, was spreading Ebola.

The three countries meet in a densely-forested region of about 1 million people. “That has been the crucible of this outbreak, where most of the cases continue to smolder and burn," Frieden said. "We’re thinking about finding new ways to reach that population more effectively.” The area also has a high incidence of malaria, and one idea to build trust may be to increase distribution of bednets to the population to protect against mosquitoes that carry malaria.

In another development Tuesday, the U.S. Department of Health and Human Services announced a $24.9 million contract with Mapp Biopharmaceutical, makers of the experimental treatment that has been pressed into emergency use during the epidemic, to speed development of the medication. Though it has never been tested on humans, the drug has been given to seven people in recent weeks, including Brantly and Writebol.

An elderly Spanish priest and a Liberian doctor who received the drug, called ZMapp, have died. Two more doctors in Liberia and a British nurse are still being treated. It is impossible to determine the medication's impact on any of them because they received the drug at different times during the course of their infections.

But in a study released Friday, researchers showed that ZMapp cured 18 monkeys infected with Ebola, including some who weren't treated until five days after they were injected with the virus.

HHS's Biomedical Advanced Research and Development Authority (BARDA) said Mapp will "manufacture a small amount of the drug for early stage clinical safety studies and nonclinical studies needed to demonstrat the drug's safety and efficacy in people." Mapp has said that its supply of the medication is exhausted.

In an email, Kevin J. Whaley, Mapp's chief executive officer, said that "BARDA's track record in advancing drugs is very impressive.  We are pleased about this collaboration to accelerate development of ZMapp."

In all, the virus has infected 3,069 people and killed more than 1,500 of them. The World Health Organization has predicted that more than 20,000 people could contract Ebola before the outbreak is quelled.

At the United Nations, Margaret Chan, director-general of the World Health Organization, also called for increased help from governments and aid groups to stem the outbreak.

“Ebola has become a global threat, which requires urgent global efforts,” Chan said before the same U.N. panel Tuesday morning. “We have no other option but to act urgently. The whole world is responsible and accountable to bring the Ebola threat under control.”

David Nabarro, a physician who is coordinating the U.N.’s response to the outbreak and recently returned from Africa, also painted a grim picture of the situation on the ground, and, like others, he said a concerted international effort could turn the tide.

"The real sense we returned with is a sense of absolute conviction that all we can do is use every single capacity we can find to get the necessary scale-up and surge of the right responses, so that this outbreak is brought back under control,” he said. “What we’ve got to do now is roll up our sleeves.”

If the international community doesn’t get a handle on the crisis, Nabarro said, it risks leaving African communities scarred by the outbreak and a sense of abandonment by the rest of the world. In addition, he said, it risks further spread of the disease to other parts of the globe.

“This could affect all of us in unexpected ways,” he said. “It’s not somebody else’s problem; it’s our collective problem.”