DAKAR, Senegal (AP) —

The United Nations human rights chief is calling for a quick investigation into death threats against Congolese Nobel Peace Prize laureate Dr. Denis Mukwege.

Michelle Bachelet on Friday said Mukwege is a “true hero" for his work as a human rights defender. The doctor founded a hospital in eastern Congo that is renowned for its work treating survivors of sexual violence.

“His life seems to be at serious risk,” she said of Mukwege, who survived an assassination attempt in 2012 and has faced death threats before.

U.N. human rights office spokesman Rupert Colville said “it’s difficult to say at this point exactly who is behind these death threats.”

He noted that Mukwege has repeatedly denounced human rights violations in Congo, adding: “The threats appear linked more to his advocacy.”

Congolese President Felix Tshisekedi has made a public commitment to ensure Mukwege’s security.