The OSCE suffered its first casualty in the three-year war in Europe's backyard after an armoured vehicle hit a landmine yesterday in the Russian-backed separatist fiefdom of Lugansk.
An unidentified US medic was killed and two others, German and Czech nationals, were wounded as they travelled on a patrol monitoring the conflict that has claimed more than 10,000 lives.
The local branch of Ukraine's SBU security service in the conflict-hit Lugansk region said in a statement that it was treating the blast as an "act of terror that led to the loss of life."
A spokesman for the Russian-backed Lugansk rebel police force told AFP that the vehicle hit an anti-tank mine.
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An AFP photographer saw the mangled remains of the vehicle - its floor ripped open and engine blown out of the hood - being hauled away from the scene for further investigation.
The incident sparked sharp words toward Moscow from both Washington and Berlin.
The US State Department's acting spokesman Mark Toner said "the incident makes clear the need for all sides - and particularly the Russian-led separatist forces - to implement their commitments" under a long-ignored peace deal signed in February 2015.
German Chancellor Angela Merkel said in a statement that "the separatists supported by Russia who illegally occupy part of Ukrainian territory by violence have a special responsibility" to respect the 2015 agreement.
Both Kiev and the West accuse Moscow of plotting and backing the fighting in reprisal for the February 2014 ouster of Ukraine's Kremlin-backed leadership.
Russia denies this and in turn accuses the US State Department of fomenting the 2013-14 street protests that ended up allying ex-Soviet Ukraine with the West.
The group said the death would not deter it continuing to operate in Europe's only war zone.
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