CHONGQING, May 8 (Xinhua) -- Four people were injured when a tanker truck under repair caught fire and exploded in southwest China's Chongqing Municipality on Tuesday.

The accident occurred at around 8:00 a.m. at a small welding workshop in Longfeng town of Beibei district on the city's western outskirts.

"The truck was said to be empty and was being repaired at the workshop," said a fireman. "But sparks from the welding ignited formaldehyde residue in its tank and caused an explosion that destroyed the two-story building."

Four people were injured, a welder and three people living on the second floor.

A witness said the damage could have been far worse because a gas station was just 30 meters from the site.

"The explosion smashed windows and might have destroyed the underground oil pipeline too," said a worker at the gas station owned by China Petroleum and Chemical Corporation (Sinopec).

Several auto repair workshops are close to the gas station, the nearest only 10 meters away, in defiance of rules that ban sparks within 25 meters from gas stations.