An American monitor with the Organization for Security and Co-operation in Europe died after a mission patrol vehicle hit a landmine in the Russian-backed separatist east, eliciting sharp words towards Moscow from US Secretary of State Rex Tillerson on Sunday.
It marked the first loss for the security body's Special Monitoring Mission (SMM) since Europe's only current war broke out more than three years ago.
The OSCE's announcement about the US monitor's death saw Kiev and the insurgents quickly trade blame over who was at fault for one of the most diplomatically sensitive episodes in a conflict that has claimed more than 10,000 lives.
The deputy head of the OSCE monitoring mission said the patrol consisted of six members who were travelling in two armoured vehicles near the village of Pryshyb in a rebel-run region of the separatist fiefdom of Lugansk.
"The explosion resulted in the death of an OSCE patrol member, a citizen of the United States," Alexander Hug told reporters in Kiev.
He also said two others -- a German and a Czech national -- were wounded and "undergoing further evaluation" in a Lugasnk hospital.
The OSCE said it would not release the names of the casualties until their families had been notified.
"We are determined to continue to implement our mandate," Hug stressed.
- 'Obstacle' to better ties -
Tillerson in a phone conversation Sunday with Ukrainian President Petro Poroshenko said Moscow's actions in eastern Ukraine remain an obstacle to improved US-Russian ties, the State Department said.
"Secretary Tillerson phoned Ukrainian President Petro Poroshenko today to discuss his recent trip to Moscow and his message to the Russian leadership that, although the United States is interested in improving relations with Russia, Russia's actions in eastern Ukraine remain an obstacle," said acting spokesman Mark Toner.
The statement said Tillerson accepted Poroshenko's "condolences" for the death of the American.
NATO Secretary General Jens Stoltenberg tweeted his reaction to the incident.
"A thorough investigation is needed of the tragic @OSCE_SMM incident in E #Ukraine. Safety & freedom of movement must be maintained," he tweeted.
German Chancellor Angela Merkel said the blast underscored the need for the warring sides to "finally respect" a long-ignored ceasefire agreement negotiated by Moscow and Kiev with the help of Paris and Berlin in February 2015.
"The separatists supported by Russia who illegally occupy part of Ukrainian territory by violence have a special responsibility here," Merkel said in a strongly-worded statement.
EU foreign affairs chief Federica Mogherini said the incident was a "reminder of the urgent need for progress on a peaceful resolution of the conflict".
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 OSCE team's 600 members in eastern Ukraine are the only independent monitoring mission in the devastated industrial war zone.
They provide daily reports on the fighting and have drawn the insurgents' ire for accusing them of being responsible for most violations of the truce deal.
- 'Anti-tank mine' -
Lugansk rebel police force spokesman Alexander Mazeikin told AFP that the OSCE vehicle "hit an anti-tank mine".
The separatists also accused the monitors of veering off the main road and travelling along an unsafe route not agreed with Russian and Ukrainian representatives.
"We know that this patrol team deviated from the main route and was moving along secondary roads, which is prohibited," the Lugansk rebels said on their news site.
Ukraine's foreign ministry branded the incident an attempt by "Moscow and its puppets to scare off OSCE monitors and to nullify efforts by Ukraine and the SMM to stabilise the situation along the front."
The Russian foreign ministry responded that it was "deeply outraged" by what it called a bid by undisclosed forces to undermine efforts to bring peace to its western neighbour.
The low-level hostilities in the European Union's backyard have been accompanied by regular casualties among 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 warned that it may take another 10 to 15 years to make the region completely safe from explosive devices.