JOHANNESBURG (AP) — Armed groups have attacked and killed Ebola response workers in eastern Congo, the World Health Organization chief said Thursday, an alarming development that could cause the waning outbreak to again pick up momentum.

“We are heartbroken that our worst fears have been realized,” Tedros Adhanom Ghebreyesus said on Twitter. He did not say how many people were killed in the attacks in Biakato Mines and Mangina. Others were wounded, he 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, comparing the environment to a war zone. Several rebel groups are active in the region.

The latest attacks come after days of deadly unrest in the city of Beni, where residents outraged by repeated rebel attacks stormed the local United Nations peacekeeping base, demanding more protection.

Ebola response work was put on lockdown in Beni, dismaying health experts who say every attack hurts crucial efforts to contain the deadly virus.

The number of cases had been dropping in the yearlong outbreak which has killed more than 2,100 people. Several days this month, zero cases were reported.

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