JOHANNESBURG (AP) — The World Health Organization's director-general says "a dear colleague" is dead after an attack on a hospital at the epicenter of a deadly Ebola outbreak.

Tedros Adhanom Ghebreyesus said in a Twitter post on Friday that "some people" are injured after the attack in Butembo.

Congo's health ministry has confirmed. It is not immediately clear who carried it out.

The response to the outbreak has been hampered by a series of deadly attacks on Ebola health centers in Butembo and elsewhere, which disrupt care and vaccination efforts and lead to a rise in new cases.

Health Minister Dr. Oly Ilunga said in a tweet that local and international health workers are courageously combating what has become the second-deadliest Ebola outbreak in history, "sometimes at the cost of their lives."