The explosion at Luhansk Regional State Administration building occurred inside the building, there was no bombing from the outside or from aircraft, Radio Svoboda has reported, referring to spokesman of the anti-terrorist operation Oleksiy Dmytrashkovsky.

"The most probable reason for the explosion was careless and unskilled use of small arms and explosives," he said on Monday.

Meanwhile, several Ukrainian media, referring to the eyewitnesses of the aforementioned accident, reported that illegal armed formations attempted to shoot down an unmanned aerial vehicle of the Ukrainian air force, but instead hit the window of the building.

Though a spokesman, the 'Luhansk People's Republic' said that the Ukrainian air force dropped a cluster bomb into the building of Luhansk Regional State Administration, and "there are deaths and injuries."