KIEV, July 6 (Xinhua) -- The Ukrainian authorities said on Friday they have opened criminal proceedings into the deadly mortar explosion earlier in the day at a military facility in the northwestern Rivne region.

The criminal case was opened on charges of violating the rules of weapon handling, the Military Prosecutor's Office in Western Region said in a statement.

Meanwhile, Yuri Tabaka, the chief military prosecutor of Rivne region, told local media that a mortar malfunction is investigated as another possible cause of the blast.

The 120-mm mortar exploded at around 05:55 a.m. local time (GMT 0255) during the live-fire tactical exercises at the Rivne combined-arms range, killing three servicemen and injuring nine others.

Three injured soldiers are in grave condition.