Mai Mai militia fighters killed four people and injured several others at two Ebola centers on Thursday in the worst yet of violence hampering efforts to tame the outbreak in eastern Democratic Republic of Congo (DRC).

Axar.az reports citing Reuters that the world’s second biggest Ebola epidemic on record has killed over 2,200 people since mid-2018, but new infections slowed in recent months.

“Ebola was retreating and now it is likely to resurge,” World Health Organization (WHO) spokesman Christian Lindmeier told a news briefing in Geneva. WHO has relocated 173 staff while the U.N. children’s agency UNICEF has evacuated 20 staff.

Mai Mai fighters and local residents have attacked health facilities sometimes because they believe Ebola does not exist and in other cases because of resentment that they have not benefited from the influx of donor funding.