BENI, Congo — Rebels have attacked and killed several health workers responding to Ebola in eastern Congo, the head of the World Health Organization said on Thursday, an alarming development that could cause the waning outbreak to regain momentum in an area that has been the site of heavy fighting.

“We are heartbroken that our worst fears have been realized,” the official, Tedros Adhanom Ghebreyesus, said on Twitter.

Three health workers were killed when Mai-Mai fighters attacked a center run by the United Nations health agency overnight in Biakato, a local official, Salambongo Selemani, told The Associated Press. One resident also was killed and Congolese forces killed one attacker and captured two others, Mr. Selemani said.

Warnings had been posted earlier demanding that the health workers leave or face “the worst,” he said.