The truck which collided with the ambulance was also gutted in the fire, they said.
The impact of the collision was such that it triggered an explosion in the ambulance's battery causing fire in the vehicle on Mhow-Nasirabad State Highway near Kesarpura village, about 15 kms from here, at around midnight yesterday, Nayagaon police inspector N S Sengar said.
There was no patient in the vehicle, he said.
As per preliminary investigation, the ambulance was apparently being used for smuggling the contraband, he said.
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Three persons in the ambulance were charred to death and one person who sustained burns was admitted to hospital, he said.
A manhunt has been launched to nab the truck driver and conductor who fled from the spot after the mishap.
An eyewitness, Shantilal Jat, a resident of Kesarpura village, said the ambulance was moving at a very high speed when the accident took place.
One lane of the highway was in-operational after a tree fell on it recently, as a result of which only one side of the two-lane road was open for traffic movement, he said.