"This appalling act is yet another reminder about the dangers associated with journalism as a profession. This killing must be immediately and fully investigated by the competent authorities," Mijatovic said in a statement circulated by the OSCE press service, Censor.NET reports citing Interfax-Ukraine.
She expressed condolences to Buzyna's family and colleagues, it said.
"I reiterate my call on the authorities to allocate all necessary resources to investigate all attacks on journalists," Mijatovic said.
She insisted also that "there must be no impunity for the perpetrators and the masterminds behind any violence against members of the media."