The outbreak has been identified as a rare type of Ebola caused by the Bundibugyo virus. Spread by bodily fluids, Ebola is highly contagious and often fatal — killing between 30% and 50% of those it infects — and causing symptoms such as fever, rash and vomiting, the organization said.

Unlike regular Ebola, Bundibugyo has no approved vaccine or treatment.

What concerns health officials and experts about this particular outbreak is that it was detected late.

Most cases are in Congo’s eastern Ituri province but it has since been found some 600 miles away in the capital, Kinshasa, and in neighboring Uganda, meaning officials do not have a clear idea of how far it might have spread.

Congo closed its land border with Rwanda on Sunday, the State Department said in a post on X.

“There are significant uncertainties to the true number of infected persons and geographic spread associated with this event at the present time,” the WHO said in a statement Sunday.

They also warn it will be difficult to fight the spread in a region that has recently seen conflict between the Congolese government and the rebel group M23, whose captured city of Goma has also confirmed one case, its local administration said.

“This is a scary one,” Jeremy Konyndyk, who led the Covid-19 response at the United States Agency for International Development, the agency since gutted by President Donald Trump, wrote on X.

Jean Kaseya, director-general of the Africa CDC, told U.K. broadcaster Sky News on Sunday that he was in “panic mode” due to a lack of medicines and vaccines as deaths rise.

The WHO’s emergency declaration means it is supporting governments and agencies’ attempts to combat the spread. The WHO Regional Office for Africa said Sunday on X that a team of 35 experts from the WHO and the Congolese Ministry of Health had arrived in Bunia, the capital of Ituri province, along with 7 tons of emergency medical supplies and equipment.

The U.S. government is helping with “surveillance, laboratory diagnostics, infection prevention and control, and other outbreak containment efforts,” the CDC said Sunday.

In a call with journalists, the CDC’s Ebola response manager Satish Pillai declined to answer repeated questions about the affected Americans, saying only that it was “assessing the needs on the ground.”

The State Department did not immediately respond to a request for comment from NBC News.

Meanwhile, the charity Doctors Without Borders, also known as Medicines Sans Frontiers, said it was “preparing to rapidly scale up our medical response” in the region.

“The number of cases and deaths we are seeing in such a short timeframe, combined with the spread across several health zones and now across the border, is extremely concerning,” Trish Newport, MSF emergency program manager at MSF, said in a statement.