Leading international health officials said Tuesday that the Ebola epidemic in West Africa is accelerating and the window for getting it under control is closing.

“Six months into the worst Ebola epidemic in history, the world is losing the battle to contain it,” Joanne Liu, international president of the medical charity Doctors Without Borders, said in a briefing at the United Nations. She faulted world leaders for failing to recognize the severity of the crisis sooner and said charities and West African governments alone do not have the capacity to stem the outbreak.

U.S. Centers for Disease Control and Prevention Director Tom Frieden, who returned Monday from a week-long trip to the countries hit hardest by the epidemic — Liberia, Sierra Leone and Guinea — said he was shocked by how rapidly the disease is spreading.

There is widespread transmission in Liberia and Sierra Leone, and a troubling spike in cases in Guinea, which until now has had more success controlling the outbreak, he said. That increase has taken place in one community where many have resisted preventive measures, such as spraying bleach, and mistakenly believe those measures are spreading Ebola.

“There is a window of opportunity to tamp this down, but that window is closing,” Frieden said. “We need action now to scale up, and we need to scale up to massive levels. . . . I cannot overstate the need for an urgent response,” he said.

Meanwhile, a U.S. missionary doctor working in Liberia tested positive for Ebola, according to the organization SIM USA. The doctor, who was not identified, was treating obstetrics patients at the organization’s ELWA Hospital in Monrovia, Liberia. He was not working with Ebola patients in the facility’s isolation unit, which is separate from the main hospital, according to the organization. He isolated himself immediately upon developing symptoms and has been transferred to the Ebola isolation unit.

SIM USA said it was unclear how he contracted the virus, which is spread through contact with bodily fluids.

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 this epidemic. Another doctor, 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 man, Patrick Sawyer, died in July after traveling to Nigeria.

The virus has infected at least 3,069 people and killed more than 1,500 of them, making it the largest and most deadly Ebola outbreak in history. The World Health Organization (WHO) has predicted that more than 20,000 people could contract Ebola before the outbreak is quelled.

Liu 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.”

President Obama, in a taped video message to the affected countries, urged residents to refrain from touching the bodily fluids of a sick person or loved one during burial. “Stopping this disease won’t be easy,” he says, “but we know how to do it.”

Health officials said the outbreak can be contained and order restored in West Africa — but only if the international community acts quickly and cooperatively.

“We have no other option but to act urgently,” said Margaret Chan, director general of the WHO. She called for increased help from governments and aid groups. “The whole world is responsible and accountable to bring the Ebola threat under control.”

Last week, the WHO outlined a six-month, $490 million plan to contain the outbreak that it said would require thousands of additional personnel.

A critical area of the epidemic involves a densely forested region where Liberia, Sierra Leone and Guinea meet, home to about 1 million people. “That has been the crucible of this outbreak, where most of the cases continue to smolder and burn,” Frieden said.

He also called on the private sector to boost its efforts. U.S. embassies are reaching out to multinational companies in the region, he said. He cited the success of one company, Firestone Liberia, in controlling the spread of Ebola among its workers.

Firestone operates on 200 acres south of Monrovia, the capital. With 7,500 employees, the company built its own Ebola treatment unit, trained staff and followed the protocol for testing, treatment and isolation. Of the 46 workers who have become infected, 31 have died, a company spokesman said Tuesday. But the company was able to identify 73 contacts of those workers and monitor them. Eleven are in isolation and are being treated, Frieden said.

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 Zmapp that has been pressed into emergency use during the epidemic, to speed development of the medication. Although it has never been tested on humans, the drug has been given to a handful of people in recent weeks, including Brantly and Writebol.

At the United Nations, Liu, of Doctors Without Borders, implored the international community to act, saying that waiting on the sidelines is not acceptable.

“Only by battling the epidemic at its roots can we stem it,” she said. “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.”

Lenny Bernstein contributed to this report.