SEOUL (Reuters) – A commercial truck carrying missile parts exploded inside a South Korean highway tunnel on Tuesday, YTN television and South Korean defense officials said.

The truck was in a four-vehicle convoy when its brakes failed on a highway linking the southern city of Taegu with Masan, just west of Pusan where a summit meeting of the Asia-Pacific Economic Cooperation forum is taking place later this month.

No casualties were reported, the Air Force said in a statement. The location was closer to Taegu on the 80-km (50-mile) highway. Officials said the tunnel was still blocked and urged drivers to find other routes.

A Taegu fire department official speaking to YTN said the fire was under control and an investigation to determine the exact cause of the accident was under way.

A chemical treatment truck was among the 33 fire engines on the scene, the fire official said.

A South Korean Defense Ministry official said by telephone that Korea Express commercial cargo trucks were moving the missile parts from an Air Force base to a location in Taegu.

The missile parts belonged to the South Korean military, another South Korean Defense Ministry official and U.S. military spokesman Kim Young-kyu said by telephone.

Along with the South”s 690,000 troops, the United States maintains more than 30,000 troops in the country.

A military source said by telephone the parts were dismantled Nike missile warheads and propulsion devices being transported to storage in Taegu.

Sparks flew from one of the brakes on the truck as it failed and caught fire in the cargo compartment of the truck, a highway patrol officer speaking to YTN said.

Another witness speaking on YTN said the trucks were marked with signs indicating explosive materials were on board.

A retired South Korean general speaking on YTN said it was unlikely a propulsion device, although loaded with a detonator and fuel, would explode simply from heat and fire.

A highway corporation official said closed circuit television inside the tunnel showed fire originating on one of the vehicles spreading to another, but it was unclear whether the second was an unrelated vehicle or part of the convoy.

Several explosions were seen from the vehicles but they were likely to be from tires and vehicle fuel tanks blowing up under heat and not missile explosives, he said.