Spokesperson for the U.S. Department of State Heather Nauert says Russian-backed mercenaries' plans to flood the Yunkom coal mine in the occupied Donbas, could threaten drinking water because of radiation.

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"Plans by Russian proxies to flood the abandoned Yunkom coal mine – site of Soviet nuclear test in 1979 – could threaten drinking water of thousands of Ukrainians in Russia-controlled eastern Ukraine. We urge Russia and its proxy authorities to act responsibly," Nauert tweeted.

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OSCE Special Monitoring Mission's observers earlier reported that the militants wanted to flood Yunkom coal mine in the town of Bunhe in eastern Ukraine, the Ukrainian online newspaper Ukrayinska Pravda wrote.

"Staff at the Yunkom mine in Bunhe (former Yunokomunarivsk, non-government-controlled, 43km north-east of Donetsk) told the SMM that some of the mine's pumps would be shut off around 14 April, and thus the mine would be gradually flooded. (In 1979, a nuclear device was reportedly detonated in a capsule inside the mine. Staff from the mine also told SMM that it is not clear how possible leakages or increased pressure from the flooding of the mineshaft could threaten the integrity of the capsule)," the report said.