Moscow’s Foreign Ministry claimed it has hard evidence to back up its charges of cold-blooded murder of civilians by Ukrainian forces deployed in eastern Ukraine to fight the pro-Russian separatists who have seized control of much of the territory there.

In a statement, the ministry referred to a pit the separatists claim they discovered at a mine located some 60 kilometers (37 miles) from Donetsk supposedly containing the bodies of civilians.

The statement added that the site was found after it was abandoned by members of Ukraine’s 25th Airborne Brigade and the Aidar Battalion.

It also referred to the discovery of a “mass grave” with corpses dressed in civilian clothes, their hands tied behind their back, indications they were shot in the head, and 9mm shell casings near the corpses.

The statement said that Moscow demands an urgent, impartial and objective investigation into the charges by the United Nations, the Organization for Security and Co-operation in Europe and the Council of Europe.

“So far, we’ve found four bodies, including one of a woman. But everything indicates that this is a mass grave,” a spokesman for the self-proclaimed People’s Republic of Donetsk told Russia’s Interfax news agency.

In Kiev, Andrei Lysenko, spokesman for the Council of National Security and Defense of the Ukraine, denied on Wednesday that Ukraine’s so-called National Guard were ever deployed in the area identified by the separatists but acknowledged that other units had been there.

The separatists on Tuesday estimated the number of pro-Russian militia and civilians killed over the past four months at 4,000. The United Nations mentions more than 3,000 fatalities since April in the eastern regions of Donetsk and Luhansk, where more than 8 million people were living before the rebellion against Kiev.