The tunnel has been closed to everyone except firefighters.
Rescue spokeswoman Magni Brandsoey said the truck driver was able to get out and call a rescue center. It was not clear how the trailer got loose.
Photos posted on Norwegian media show a huge black billow of smoke in the mountainous area in western Norway.
The explosion happened around 11 am (0900 GMT) in a 1,901 meter (6,237 feet) subsea tunnel linking two islands in the Sogn og Fjordane county.