"The OSCE has launched an investigation into the incident and expects maximum co-operation from all concerned," Zannier said, as quoted by the organization's statement.
He also called on the parties to the conflict to ensure safety for the OSCE monitors.
"This appalling incident underlines the increasingly dangerous security environment the SMM monitors are working in every day. All sides are responsible for the safety and security of the OSCE monitors. The Mission must have universal support and everyone must recommit to its objectives and to protecting the monitors from danger," Zannier said.
According to the Russian Foreign Ministry, the killed observer was a US citizen. The nationality of the killed was later confirmed by Principal Deputy Chief Monitor of the OSCE SMM to Ukraine Alexander Hug.
