An American member of the Organization for Security and Co-operation in Europe (OSCE) was killed during a mission in eastern Ukraine on Sunday.
This marks the first time a member of a Special Monitoring Mission has been killed while on duty, OSCE Secretary General Lamberto Zannier said in a statement.
Two other monitors were injured when their vehicle went over a mine in the non-government controlled Luhansk region.
"I am deeply shocked and saddened by this tragic death," Zannier said in a statement. "This appalling incident underlines the increasingly dangerous security environment the SMM monitors are working in every day. All sides are responsible for the safety and security of the OSCE monitors."
The blast, which was likely caused by a mine, happened at 11:17 a.m. Sunday morning, OSCE Principal Deputy Chief Monitor Alexander Hug told CNN.
Austrian Foreign Minister Sebastian Kurz called for a thorough investigation into the incident and for those responsible to be held accountable.
"All circumstances of this tragic incident must be clarified without hindrance," Kurz said in a statement. "I urge all actors that have an influence on the situation on the ground to live up to their responsibility in guaranteeing safety and security to the SMM."
Tensions between Ukraine and Russia remain high three years after the annexation of Crimea. In addition, a 2015 ceasefire agreement signed by the countries is frequently violated.
Monitors are responsible for verifying the withdrawal of weapons noted in the ceasefire agreement.