The Organization for Security and Cooperation in Europe said Sunday one of its staff was killed after an observer mission patrol vehicle hit a landmine in rebel-held east Ukraine.
It marked the first loss for the security body's Special Monitoring Mission (SMM) in Ukraine since Europe's only war began more than three years ago.
"Tragic news from Ukraine: SMM patrol drove on mine. One OSCE patrol member killed, one injured," Austrian foreign minister and the OSCE's current chairman Sebastian Kurz wrote on Twitter.
An OSCE official in Kiev said the group could not disclose the person's nationality because their next-of-kin were still in the process of being notified.
The incident occurred near the village of Pryshyb in the Russian-backed rebel fiefdom in the eastern Lugansk region.
"Obviously, the blast was strong enough to penetrate an armoured vehicle," the OSCE official told AFP. "All of our vehicles are armoured."
The OSCE gave no details on the condition of the wounded staff member as Kurz demanded a "thorough investigation."
"Those responsible will be held accountable," Kurz wrote.
A statement issued by the Lugansk separatists on their official news site said the OSCE team had veered off the main road and was travelling along an unsafe route.
"We know that this patrol team deviated from the main route and was moving along secondary roads, which is prohibited by the OSCE Special Monitoring Mission mandate," the separatist statement said.
"We have repeatedly drawn the OSCE SMM's attention to the need to follow security measures while travelling on its monitoring missions."
The insurgents' claim could not be immediately confirmed.
The war between Kiev's forces and the Moscow-backed eastern insurgents has claimed the lives of more than 10,000 people and driven more than two million from their homes since 2014.