Members of a Red Cross team head out May 14 to look for suspected victims of Ebola in Mbandaka, Congo. (Karsten Voigt/International Federation of Red Cross and Red Crescent Societies/AP)

 The World Health Organization said Friday that criteria had not yet been met to declare a public health emergency of international concern in Congo, where three cases of Ebola have now been confirmed in an urban area.

The outbreak is the most serious since an epidemic in West Africa outpaced an international response between 2014 and 2016, infecting more than 28,000 people and killing more than 11,000.

“This situation can yet be brought under control,” Robert Steffen, chairman of the WHO’s emergency committee, said at an emergency meeting of experts called by the organization in Geneva. But the international community must sustain a “vigorous response” to prevent a sharp deterioration, he added.

Eleven cases of the Ebola virus were confirmed in northwest Congo on Friday, including two more in the bustling river port of Mbandaka, where health officials say a growing outbreak could see an “explosive increase.” The first urban case was confirmed in the city of more than 1 million Thursday, nine days after the outbreak was officially declared. A total of 14 cases have now been confirmed, according to Congo’s Ministry of Health.

The WHO was accused of responding slowly in the early days of the two-year epidemic that began in 2014, and the organization has taken pains to ensure it is acting more quickly — and is seen as doing so — this time.

The WHO has put Congo’s nine bordering countries on high alert, and the International Organization for Migration is aiding the Congolese government in deploying teams of epidemiologists and other medical staff to 16 border crossings. Both organizations are arms of the United Nations. Doctors Without Borders, known internationally by its French acronym, MSF, and Alima, another nongovernmental organization, are among others leading ground efforts to contain the outbreak.

Though only 25 people have died so far from suspected Ebola cases, health officials are worried that the virus has had ample time to spread into remote and hard-to-reach areas of the rain forest that densely covers much of the affected region. 

The WHO is sending 7,540 doses of an experimental vaccine, more than half of which have already arrived in Congo’s capital, Kinshasa. The vaccine, yet unlicensed, has been cleared for “compassionate use” by the WHO, and has proven effective in trials. The lack of licensing presents a distinct challenge: Each recipient is required to provide fully informed and signed consent. This means surmounting significant language barriers in a region with many dialects. Additionally, the vaccines must be transported and stored at between minus-60 and minus-80 degrees Celsius (minus-76 to minus-112 Fahrenheit), a tall order in a humid rain forest with little infrastructure.

The vaccine will be issued, starting Sunday, to those identified as having been in contact with suspected or confirmed cases to try to create what are known as “rings” of immunity. The supplies will be enough to vaccinate 50 rings of 150 people, the WHO said. More vaccines would be needed to fully reach the 15,527 such people who have been identified so far. 

The epicenter of the outbreak is more than 100 miles south of Mbandaka, in Bikoro, where a dozen cases have been confirmed. Mbandaka lies on the eastern bank of the Congo River, Africa’s second-longest after the Nile. Tens of millions of people live along the river, and the capitals of Congo, the Central African Republic and Congo Republic lie along it and its tributaries.

Health officials are increasingly worried that the virus could be conveyed quickly over long distances by boat between the towns that dot the river’s banks, or to Kinshasa by one of the many who travel and trade between the two cities. Ebola reaching the capital city of more than 14 million people is a nightmare scenario for containment.

Ebola is not the most contagious of diseases, but it is unusually fatal. Once contracted, it disables parts of the immune system, resulting in fever, headaches, vomiting, diarrhea, dehydration, and ultimately organ failures and internal bleeding. Because it can take a week or longer before symptoms are noticeable, a person can travel long distances and have contact with many others by the time they are diagnosed. And as the symptoms are similar to those of malaria, also endemic to Congo, it is even more difficult to trace. 

In the 2014 West Africa epidemic, the U.S. government and military played a key role in eventually containing the disease’s spread. President Barack Obama asked Congress for nearly $1 billion in emergency funding for the CDC and deployed 3,000 armed-forces personnel. President Trump has called for the $252 million that remained in that fund to be cut, and on May 8, the same day that the current outbreak in Congo was declared, the White House’s top official responsible for pandemic response quit abruptly, apparently as part of a reorganization by new national security adviser John Bolton.

So far, the WHO has refrained from calling on travel or trade restrictions in the affected area. 

“WHO continues to monitor travel and trade measures in relation to this event, and currently there are no restrictions on international traffic in place,” the organization said in a statement.

Lena H. Sun in Washington contributed to this report.