It is the first victim of the OSCE Special Observation Mission (MSS) in Ukraine since its deployment more than three years ago in the region, where it is responsible for ensuring compliance with the peace agreements.
"The explosion caused the death of a member of the OSCE patrol, a citizen of the United States," said the deputy head of the OSCE MMS in Ukraine, Alexander Hug, during a Press conference in Kiev.
Two other observers, German and Czech, were injured in the explosion and hospitalized, he said. Their names will be unveiled once their families have been informed, according to the OSCE.
"We are determined to continue to fulfill our mandate," Hug said.
At the time of the explosion, according to the Ukrainian army, there were no clashes due to a temporary truce set up by the rebels and Kiev on 1 April.
The United States was quick to react with Secretary of State Rex Tillerson, who, in a telephone interview with Ukrainian President Petro Poroshenko, said Moscow's action in eastern Ukraine remains an "obstacle" to an improvement in relations between the US and Russia.
In this call, the two leaders also stressed "the need for all parties, and in particular the separatist forces led by Russia, to implement immediately their commitments under the Minsk peace accords, The spokesman of the American diplomacy, Mark Toner.
Washington also urged Russia to use "its influence on the separatists to allow the OSCE to conduct a transparent, accurate and full investigation."
Austrian Foreign Minister and OSCE Chairman Sebastian Kurz called for an "in-depth investigation" and said that "accounts will be requested from officials".
For his part, Ukrainian President Poroshenko expressed his condolences to the relatives of the victim, assuring that "this crime must absolutely be clarified and those responsible for it must be punished."
German Chancellor Angela Merkel called on "the parties to the conflict to undertake everything to determine what has led to and is responsible for this tragic incident".
This tragedy "shows once more how unstable the situation in eastern Ukraine is," said French Foreign Minister Jean-Marc Ayrault.
Some 600 OSCE observers are responsible for monitoring compliance with the peace accords in the rebel eastern Ukraine where more than 10,000 people have been killed since the outbreak of the conflict in April 2014.
- 'Cynical act' -
The OSCE patrol jumped on a mine near the village of Prychyb in the separatist region of Lugansk.
"The explosion was powerful enough to puncture an armored vehicle," said an OSCE official in Kiev, adding that all vehicles in the mission were armored.
It was an "anti-tank mine", said the police official of the Lugansk region, Alexandre Mazeïkine, to AFP.
In a statement, the separatists in Lugansk claimed that the OSCE patrol had "diverted from the main road and was traveling on secondary roads, which is prohibited by the MMS mandate". This information could not be confirmed immediately.
Ukrainian diplomacy has accused Moscow and its puppets of trying to intimidate OSCE observers and undermine efforts to stabilize the situation along the demarcation line between the territories controlled by Kiev and Ukraine. the rebels.
Russia is accused by the Ukrainian authorities and Westerners of militarily supporting the Ukrainian rebels, which Moscow strongly denies.
The Russian Foreign Ministry said it was "outraged by the cynical act ... against international observers", denouncing a "very probable provocation aimed at undermining the process of settling the conflict" in the east of the country. 'Ukraine.
The Ukrainian Ministry of Defense announced on Thursday that it had defused 150,000 mines since the beginning of the armed conflict in the east in April 2014, while specifying that only 3,000 hectares of 700,000 hectares had been completely cleared of all kinds of mines.
The total demining of this region could take between 10 and 15 years, according to the ministry.