The World Health Organization said Friday that security concerns in the Democratic Republic of the Congo’s North Kivu region were preventing aid workers from reaching certain areas — and leaving open the possibility of the Ebola virus spreading.

At least 1,500 people could be exposed to the virus, WHO spokesperson Tarik Jasarevic told reporters in Geneva, according to Reuters.

Congo’s health ministry declared an outbreak of Ebola on Aug. 1 in the North Kivu region. As of Wednesday, the WHO reports 51 confirmed cases and 27 probable cases of Ebola in the region, with 44 people (17 confirmed, 27 probable) having died of the disease.

“We don’t know if we are having all transmission chains identified. We expect to see…