Lax safety standards led to the powerful explosion at a factory in the eastern Chinese city of Nanjing last week, China's safety office said Sunday.

A leak of propylene gas was the direct cause of the accident that killed 13 people, the Work Safety Committee Office of the State Council said in a report.

The explosion ripped through an abandoned plastics factory in Nanjing's Qixia district at 10:11 am Wednesday, leaving buildings within 100 meters seriously damaged. A bus near the site of the explosion was destroyed by the ensuing fire.

As of Saturday, 13 people are confirmed dead while 120 are injured. Four of the injured are in critical conditions.

The blast was caused by the gas leak that started at 9:30 am, after workers damaged an underground propylene pipeline while using excavators, said the office.

The propylene evaporated and encountered an open flame, triggering the explosion.

The excavation work proceeding without proper safety standards was the reason for the explosion, the office said, calling for more attention to be paid to workplace safety.