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The damaged vehicle that drove over a mine while transporting members of the Organization for Security and Cooperation in Europe (OSCE), who were killed and injured from the incident on Sunday, is moved from the scene in Luhansk region, Ukraine, April 23, 2017. REUTERS/Alexander Ermochenko

A U.S. citizen patrolling with the Organization for Security and Co-operation in Europe (OSCE) was killed in an explosion in eastern Ukraine’s Luhansk region Sunday, the U.S. Department of State said.

“The United States is shocked and deeply saddened by the death of a U.S. citizen serving as a paramedic with the Organization for Security and Cooperation in Europe (OSCE) Special Monitoring Mission (SMM), when his vehicle struck an explosive in separatist controlled territory in eastern Ukraine,” a statement released by the state department said.

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“We extend our deepest and heartfelt condolences to his family, friends, and loved ones, and we wish a quick recovery for those injured in the blast,” it added.

The explosion, which took place at 11:17 a.m. local time (4:17 a.m. EDT) Sunday morning, was the first instance where a member of the OSCE Special Monitoring Mission was killed while on duty. The identity of the victim has not been released.

Two other monitors from Germany and the Czech Republic were also injured in the blast.

“This appalling incident underlines the increasingly dangerous security environment the SMM monitors are working in every day,” OSCE Secretary General Lamberto Zannier said. “All sides are responsible for the safety and security of the OSCE monitors.”

According to the Associated Press, the explosion was a result of a land mine blowing up and the OSCE’s current chairman Austrian Foreign Minister Sebastian Kurz said an investigation must be carried out into the blast, adding that “those responsible will be held accountable.”

“The United States again calls upon Russia to use its influence with the separatists to take the first step toward peace to eastern Ukraine and ensure a visible, verifiable, and irreversible improvement in the security situation,” the state department statement added.