Two Air Force Tornado jets on a training mission crashed into each other, as rescuers make a desperate search for the four Italian pilots who ejected.
A witness told TV station Sky TG24 he saw "an enormous ball of fire that lasted about five seconds" before the planes crashed into the hillside, near the town of Mozzano.
Local mayor Guido Castelli stated that no civilians were injured.
A firefighting helicopter was trying to extinguish the blaze. Flames and dark smoke are still visible for miles in the region.
One witness told Il Fatto Quotidiano: "We heard a tremendous roar and when we looked up to the sky we saw pieces of plane falling."
A spokesman for the Italian Air Force said rescue helicopters and other aircraft were searching for the missing crew members.
A team of experts had been sent to the crash site to investigate what happened. Some eyewitness reports from the scene suggested the two aircraft had touched wings while flying at low altitude, according to Defence News.
The Tornado jet has a top speed of more than 1,600mph and can fly up to 2,420 miles without refuelling.
They are used by a number of air forces around the world, including Italy, Britain and Germany.