The OSCE said Friday it was limiting its patrols of Ukraine's eastern war zone after the death of a US paramedic in an apparent mine blast last weekend.
The deputy head of the Organization for Security and Co-Operation in Europe (OSCE) monitoring mission in Ukraine said the April 24 incident was "not an accident".
But he stopped short of assigning blame for the security watchdog's first death in the former Soviet republic since Kiev's campaign against Russian-backed insurgents began in April 2014.
"At the moment we only patrol on solid ground, which means asphalted or concrete roads," Alexander Hug said in a statement.
"This will, of course, restrict our abilities to visit certain locations," he said. "As a result there will be fewer facts monitored and reported."
An unarmed team of three was travelling along a dirt road in an armoured vehicle when it hit what a separatist security chief told AFP was an anti-tank mine.
Hug said the monitors had driven along the same road two hours earlier in the rebel-run region and were making their return trip when the blast occurred.
"Let me be clear with you -- what happened on Sunday was not an accident. It was the use of an indiscriminate weapon. An indiscriminate weapon that should never have been there in the first place," Hug said.
The separatists had previously claimed that the OSCE patrol had veered off an itinerary agreed with Russia and Ukraine.
Hug called the claim "disgraceful".
"Our mandate is absolutely clear. We are meant to have full and unhindered access everywhere with no exceptions," he said.
The OSCE mission has been monitoring the implementation of a largely-ignored peace deal the warring sides signed in February 2015.
It is the only organisation that provides daily reports from the front about ceasefire violations and progress in the sides' promise to withdraw their arms.
Ukraine's military on Friday reported the death of one of its soldiers in another mine blast.
The hostilities in eastern Ukraine have been accompanied by regular casualties among civilians and fighters who either stepped or drove on explosive devices.
Ukraine's defence ministry said last week it had defused 150,000 such devices since the start of the war.
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 mines.