Seven people have been killed in a military helicopter crash in the Serbian capital of Belgrade, Xinhua news agency reported Saturday.Four pilots, two medical staff and a one-week-old baby died Friday night when an army helicopter crashed near the Belgrade's airport due to severe weather, Serbian Prime Minister Aleksandar Vucic told a government session on Friday.The cabinet has voted to make Sunday a national day of mourning.The pilots were sent to the southern region of Raska to help transport the baby, who was in a critical health condition, after an ambulance carrying the baby was stranded due to a landslide blocking the road, government officials revealed at a press conference.Brig. Gen. Predrag Bandic told journalists that the flight was operated by experienced pilots with over 1,000 hours of flight, and the MI-17 transport helicopter was one of Serbia's best.