One member of European security watchdog OSCE’s monitoring mission in eastern Ukraine was killed and another injured after their vehicle drove over a mine near Luhansk.
An American man was killed and a German woman was injured on Sunday morning, a spokesman for Austria's foreign ministry said.
Austria holds the rotating presidency of the Organization for Security and Cooperation in Europe (OSCE).
Three years after Moscow annexed the Crimean region, tensions between Ukraine and separatists in the Russian-held eastern part of the country remain high and a 2015 ceasefire agreement is violated regularly.
The Ukrainian military said the incident took place at 10:17 local time (0717 GMT) near the small village of Pryshyb, which is controlled by pro-Russian separatists.
The unarmed, civilian OSCE mission with more than 700 international observers was deployed in 2014. The role of the monitors includes verifying the withdrawal of heavy weapons as agreed under the 2015 ceasefire agreement.
“Tragic news from #Ukraine: SMM patrol drove on mine. One #OSCE patrol member killed, one injured,” Sebastian Kurz, the Austrian foreign minister and the OSCE’s current chairman, wrote on Twitter.
Kurz also called for an investigation of the incident and, in another tweet, said he had spoken to the mission’s ambassador, Ertugrul Apakan.
"Need thorough investigation; those responsible will be held accountable,” he said on Twitter.
A spokesman for the OSCE said further information would be released as it became available.
The 57 member states of the OSCE, which include Ukraine, Russia and the United States, in March extended its monitoring in Ukraine by a year.