Rebels have attacked and killed Ebola response workers in eastern Congo, the World Health Organization chief said yesterday, an alarming development that could cause the waning outbreak to again pick up momentum in what has been called a war zone. "We are heartbroken that our worst fears have been realized," 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, local official Salambongo Selemani told The Associated Press. One resident also was killed and Congolese forces killed one attacker and captured two others, Selemani said. Warnings had been posted earlier demanding that the health workers leave or face "the worst," the official said.

This is not the first time that health workers trying to contain the second-worst Ebola outbreak in history have been targeted. Some have called this outbreak more complicated than any other. Several rebel groups are active in the region, and local officials say some believe Ebola is nothing but a political ploy.

In a second attack, Allied Democratic Forces rebels killed a civilian overnight in Mangina, a rural community in Beni territory, territory administrator Donat Kasereka Kibwana said. The target was not the Ebola center but the local population, the official said.

The latest attacks come after days of deadly unrest in the city of Beni, where residents outraged by repeated rebel attacks stormed the local U.N. peacekeeping base, demanding more protection. The WHO evacuated 49 of its staffers there, leaving 71 in place.