April 23 (UPI) -- An American observer was killed and two European monitors wounded Sunday in an explosion in eastern Ukraine, the Organization for Security and Co-operation in Europe said.
Alexander Hug, OSCE's principal deputy chief monitor for the unarmed civilian mission, said the blast was most likely caused by a land mine.
The paramedic served as a patrol member of the OSCE Special Monitoring Mission to Ukraine. The organization did not release his identity. It was the first time an SMM patrol member killed on duty, said OSCE Secretary General Lamberto Zannier.
Two other monitors, a German and a Czech national, were injured in the explosion. It occurred in the Luhansk region, a volatile area rocked by Russia-backed separatists and Ukrainian armed forces.
"This appalling incident underlines the increasingly dangerous security environment the SMM monitors are working in every day," Zannier said. "All sides are responsible for the safety and security of the OSCE monitors."
On Twitter, NATO Secretary-General Jens Stoltenberg called for an investigation into the monitor's death.
He told Ukrainian President Petro Poroshenko in a phone conversation Sunday that Moscow's actions in eastern Ukraine remain an obstacle to improved ties, the State Department said.