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," Austrian foreign minister and the OSCE's current chairman Sebastian Kurz wrote on Twitter.
The OSCE mission added later that "two members have been taken to hospital for further examination" but did not give details on their condition.
An official from the organisation in Kiev said they could not disclose the victims' nationalities because their next-of-kin were still in the process of being notified.
The incident occurred close to Ukraine's volatile frontline near the village of Pryshyb in the Russian-backed eastern rebel fiefdom of Lugansk.
"Obviously, the blast was strong enough to penetrate an armoured vehicle," the OSCE official told AFP. "All of our vehicles are armoured."
Group chairman Kurz demanded a "thorough investigation" into the incident and insisted that "those responsible will be held accountable".
The OSCE team's 600 members in eastern Ukraine comprise the only independent monitoring mission in the industrial war zone. They provide daily reports on fighting and have drawn criticism from the warring sides.
Lugansk rebel police force spokesman Alexander Mazeikin told AFP that two OSCE vehicles were travelling in the rebel-run region when "one of them hit an anti-tank mine".
A statement issued by the Lugansk separatists on their 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," the separatists 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.
Ukraine's military said no fighting was occurring in the area because the sides were honouring a temporary truce agreement that went into effect on April 1.
"At the time of the explosion, the ceasefire was being observed by both the Ukrainian forces and the illegal rebel groups," Kiev's armed forces said in a statement on Facebook.
The war between Kiev's pro-Western troops and the Moscow-backed insurgents has claimed the lives of more than 10,000 people and driven at least two million from their homes since 2014.
Daily casualties have fallen substantially since the height of the fighting in the industrial heartland but sporadic clashes drag on as a peace plan has stalled.
But the low-level hotilities have been accompanied by regular casualties from civilians and fighters either stepping or driving on mines.
Ukraine's defence ministry said Thursday it had defused 150,000 explosive devices since the war began.
But it added that only 3,000 of the 700,000 hectares (7,400 of the 1.7 million acres) along which the war is being waged had been cleared of the various types of landmines.
It added that it may take another 10 to 15 years to make the region completely safe from explosive devices.
Both Kiev and the West accuse Russia of plotting and backing the conflict in reprisal for the February 2014 ouster of Ukraine's Kremlin-backed leadership.
Moscow flatly denies this and accuses Washington of plotting the 2013-14 street protests that ended up allying ex-Soviet Ukraine with the West.