An American member of the OSCE’s monitoring mission in eastern Ukraine was killed after their vehicle was blown up by a mine.

Another member, a German woman, was wounded in the blast that took place in the Luhansk region.

Austrian Foreign Minister Sebastian Kurz, who holds the rotating chairmanship of the Organization for Security and Cooperation in Europe, called for an investigation of the Sunday blast and said on Twitter that “those responsible will be held accountable.”

He said one mission member was injured, while officials of the Russia-backed separatist rebels, who hold much of Luhansk, said either one or three were injured.

He said he had spoken to the mission’s ambassador, Ertugrul Apakan.

The Luhansk rebels’ self-proclaimed security ministry said the mine had been laid by Ukrainian forces that have been fighting the rebels for three years in a war that has killed more than 9,900 people.

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.

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.