Three caravans and a car parked off the Coast Road in Naxxar caught fire early on Tuesday.
The police said they were informed of the incident at 2.45am. Several explosions were reported.
Firemen from the Civil Protection Department went on site and controlled the fire. No one was injured.
The vehicles were parked close to il-Bajja taż-Żwiemel.
The source of the explosions is not known.