Belgrade, Serbia: A Serbian military helicopter evacuating a five-day-old baby for emergency medical treatment has crashed in heavy fog, killing all seven people on board, the officials said Saturday.

A four-member crew, two medical workers and the baby were all killed in the crash, Prime Minister Aleksandar Vucic told RTS television.

The incident was the worst of its kind since 1996, Serbian army chief of staff Ljubisa Dikovic said.

The Mi-17 helicopter transporting the baby who was suffering severe respiratory problems to a Belgrade hospital crashed late Friday near the Serbian capital, the defence ministry said in a statement.

The baby was initially being rushed to hospital in an ambulance, but the vehicle got stuck in a mudslide in central Serbia some 200 kilometres (120 miles) south of Belgrade.

Air traffic controllers lost contact with the helicopter while it was awaiting clearance to land. The aircraft disappeared from the radar a few minutes later, Vucic said.

The helicopter had attempted to land twice due to poor visibility, the helicopter unit commander Predrag Bandic told reporters.

Local media reported that several commercial flights were redirected to other airports due to heavy fog. Shortly before the accident lights at two of Belgrade's main football stadiums -- of the Red Star and Partizan clubs -- were reportedly lit so the helicopter could try to land there.

‘The weather conditions were complex but nothing that such an experienced crew in a helicopter equipped as this one could not handle,’ Bandic said.

According to military officials, the pilot was among the most experienced in the Serbian army, with more than 4,000 hours of flight.

‘The crew was the best that the Serbian army had to offer for this task, (and) the helicopter was fully functional before taking off,’ Bandic said.

The government declared Sunday a day of mourning.

Military police have cordoned off the crash site near the airport.