An American member of the Organization for Security and Cooperation in Europe was killed Sunday after hitting a landmine while on patrol in eastern Ukraine, the first casualty of the monitoring group during its time observing the country’s conflict. His name has not yet been released, but he was described as a paramedic. Two other monitors, a German and a Czech national, were also injured in the incident, which occurred in rebel-controlled Luhansk. Despite a 2015 ceasefire agreement, fighting has continued in the area and the OSCE has regularly complained of hindered access and harassment from the Russian-backed separatists. In response to the incident Sunday, the U.S. State Department called on Russia to “use its influence with the separatists to allow the OSCE to conduct a full, transparent, and timely investigation,” spokesman Mark Toner said in a statement. He added that U.S. Secretary of State Rex Tillerson spoke with Ukrainian President Petro Poroshenko, telling him that “Russia’s actions in eastern Ukraine remain an obstacle” for the U.S. in mending ties with Moscow.