The Ebola outbreak in eastern Congo “continues to outpace the response efforts,” the World Health Organization’s emergencies chief said on Tuesday after returning from a trip to Bunia, in Ituri province, which is one of the worst hit areas.

“Perhaps the most alarming finding is that many of the newly reported deaths are people who died in their communities without ever reaching a health facility and without receiving care,” Chikwe Ihekweazu said. “And as of today, 80% of new cases are outside our contact lists and so are coming to us from unknown chains of transmission.”

Congo has been battling the outbreak of a rare type of Ebola since May with no approved treatment or vaccine. The Africa Centre for Disease Control said last week that it is the fastest-growing Ebola outbreak on the continent.

At least 1,926 people have been infected and 702 died in three provinces in Congo from the rare Bundibugyo virus, Congolese authorities said. Cases have also been confirmed in neighbouring Uganda.

Last week, the U.S. Centres for Disease Control and Prevention said a U.S. citizen working for a humanitarian organization in Congo has tested positive for the Ebola virus, without providing further details.

Ihekweazu said that despite “our best efforts … we have not caught up in the race.”

The response has been hampered by a funding gap, attacks on health centres, and an ongoing conflict in eastern Congo, as well as mistrust among local communities.

On Monday, dozens of people working at an Ebola virus treatment centre in northeast Congo went on strike over unpaid salaries and bonuses.

Clinical trials for treatment began last week after researchers launched a highly anticipated study in the hope of fighting the virus.