Gunmen killed four health workers in two attacks on World Health Organization camps responding to an ebola outbreak in the war-torn Democratic Republic of Congo, as violence escalates and threatens efforts to halt the world’s second-deadliest bout of the virus.

The nighttime assaults, in the busy gold-mining town Biakato and the nearby town of Mangina, mark the most deadly day for Ebola responders since the outbreak was declared in August 2018. The WHO confirmed the fatalities—including a member of a vaccination team, a police...