Temperatures soared so high in the blaze that the road's tar melted underfoot, rescuers told France-2 television. French television showed pictures of blackened, soot-coated shells of two tractor-trailers in the tunnel.
The Frejus tunnel is a major thoroughfare linking the French city of Lyon with Turin, Italy, accounting for four-fifths of commercial roadway traffic between the countries.
Most of the truck traffic — about 3,800 trucks per day on average — likely would be rerouted through the Mont Blanc tunnel, requiring a 124-mile detour.
The fire began when a truck carrying tires burst into flames. Two Slovenian truck drivers in their early 20s died, French police said.
Coordination between the Italian and French firefighters was "great," Italian Transport Minister Pietro Lunardi said after meeting at the scene with his French counterpart, Dominique Perben.
"This accident will help to improve the level of security," Lunardi told Sky TG24 news.
He added that security was boosted in the tunnel after a 1999 Mont Blanc tunnel fire, but "it can be increased" further.
The Mont Blanc inferno burned for two days while firefighters tried to reach trapped cars. That blaze killed 39 people.
Lunardi said a French magistrate would lead a technical probe into the cause of the blaze, which burned six vehicles — the tire truck, a truck transporting glue, two other tractor-trailers and two fire vehicles.
French authorities said it was too early to tell whether the tunnel would remain shut during February's Winter Olympics in Turin, but Lunardi was hopeful it would reopen soon.
"From what they've told me, the damage isn't as serious as had been thought at first," Lunardi said, according to the ANSA news agency.