(Reuters) - An American paramedic working for European security watchdog OSCE’s monitoring mission in eastern Ukraine was killed and two others were injured on Sunday when their vehicle struck a mine, triggering a U.S. call for a transparent, timely investigation.

A woman walks past vehicles of the Organization for Security and Cooperation in Europe (OSCE) special monitoring mission for Ukraine parked near the OSCE office, in the pro-Russian rebel-held city of Luhansk, Ukraine, April 24, 2017. REUTERS/Alexander Ermochenko

The Organization for Security and Cooperation in Europe (OSCE) said the killing was the first death of one of its members while on patrol in Ukraine, where more than 700 international observers help report on a simmering conflict that has deeply strained relations between Russia and the West.

A 2015 ceasefire between Ukraine and Russian-backed separatists in the eastern part of the country is regularly violated, and Washington cites the conflict as a key obstacle to improved relations between Russia and the United States.

U.S. State Department spokesman Mark Toner said the killing underscored the increasingly dangerous conditions under which the OSCE mission operated, including grappling with “access restrictions, threats, and harassment.”

“The United States urges Russia to use its influence with the separatists to allow the OSCE to conduct a full, transparent, and timely investigation,” Toner said.

U.S. Secretary of State Rex Tillerson also spoke about the incident on Sunday with Ukrainian President Petro Poroshenko, who offered his condolences.

“This tragic incident makes clear the need for all sides - and particularly the Russian-led separatist forces - to implement their commitments under the Minsk agreements immediately,” Toner said.

The Minsk peace agreement, brokered by France and Germany and signed by Russia and Ukraine in February 2015, calls for a ceasefire, the withdrawal of heavy weapons from the front line and constitutional reform to give eastern Ukraine more autonomy.

But since the deal the sides appear stuck in a stalemate broken periodically by sharp resurgences of fighting that Kiev and the Kremlin accuse each other of instigating.

Tillerson told Poroshenko that although Washington wanted better ties with Moscow, “Russia’s actions in eastern Ukraine remain an obstacle,” the State Department said.

GERMAN, CZECH MONITORS WOUNDED

The Ukrainian military said the blast took place at 10:17 local time (0317 ET) near the small village of Pryshyb, which is controlled by pro-Russian separatists.

The OSCE said in addition to the death, two of its monitors were taken to the hospital.

Slideshow ( 4 images )

An Austrian foreign ministry spokesman said a German woman was injured. Austria holds the rotating presidency of the OSCE.

Alexander Hug, the Special Monitoring Mission’s Principal Deputy Chief Monitor, told reporters that the second injured person was from Czech Republic.

German Chancellor Angela Merkel said she was appalled and her government expected the parties to investigate and determine who was responsible.

“The conflict parties must also finally implement the long-agreed ceasefire,” Merkel said in a statement. “And there, the Russia-supported separatists, who are illegally and violently occupying parts of Ukrainian territory bear a particular responsibility.”

The Russian foreign ministry said the circumstances pointed to likely provocation aimed at undermining the peace process. It urged the parties to the conflict to intensify direct talks.

“We’re deeply outraged by this cynical action, which led to the loss of human life and was directed against international observers who work for the sake of peace,” the ministry said in a statement on its website.

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.