The World Health Organization (WHO) said Wednesday that an outbreak of the deadly Ebola virus in two eastern provinces of Congo does not rise to the level of a global public health emergency.

But the group said it is increasingly concerned about insecurity in an area where armed rebel groups operate freely, threatening public health workers who are racing to contain the spread of the virus.

“This is extraordinary because of a particular challenge with respect to security of armed groups attacking particularly also in the area of the outbreak,” Robert Steffen, who chairs the WHO’s emergency committee tasked with overseeing the current outbreak, said at a news conference Wednesday in Geneva.

ADVERTISEMENT Tedros Adhanom Ghebreyesus, the WHO’s director-general, said 20 armed groups are operating in North Kivu Province, the area at the heart of the outbreak that began in late July.

The Congolese Health Ministry said Wednesday that 220 people had been infected with the Ebola virus in North Kivu and neighboring Ituri provinces. Of those, 142 people have died.

After the number of cases started to decline in August and early September, new chains of transmission began last month after a bombing in Beni, a trading post of about a quarter-million residents near the border with Uganda. The bombing, blamed on Islamist rebels from across the border, led to several days of mourning in which health response efforts came to a virtual halt.

Security concerns have already caused the Centers for Disease Control and Prevention (CDC) to pull several responders from the North Kivu region. A CDC spokesman said four Americans — three from CDC and one from the U.S. Agency for International Development — suspended their operations after an attack on a Congolese Army base in late August.

But WHO officials said they were not yet sufficiently alarmed by the prospects of the virus spreading across international borders.

“What we decided today is based on the situation on the ground,” Tedros said.

There are still 250 WHO personnel deployed on the ground, and groups like Doctors Without Borders are also operating Ebola treatment centers in the region. Several other nongovernmental organizations are operating teams that conduct safe burials and vaccinations of those who are at risk.

More than 18,000 people, including 4,000 children, have received the vaccine, Tedros said.

The WHO has only declared four Public Health Emergencies of International Concern since the designation was created in 2005. The 2009 outbreak of swine flu, outbreaks of polio and the Ebola virus in 2014 and the global spread of the Zika virus in 2016 all met the criteria for an emergency of international concern.

Some public health experts warned that the WHO’s decision should not be read as a sign that the outbreak is under control.

“The situation is getting worse — not better — and the risk of spread to adjacent countries seems very high,” said Ron Klain, who led the American response to the Ebola outbreak in three West African nations in 2015. “The most urgent need is to figure out a security solution that would allow CDC experts to function in the impacted region instead of being relegated to the sidelines.”

Updated: 2:24 p.m.