Police have revealed that workers accidentally caused the Shanghai Telecom Building fire which killed at lease four people Tuesday afternoon.

Preliminary investigations showed sparks from a metal cutting machine used by workers to cut an air conditioning duct had ignited thermal insulation material wrapped around the pipe.

Five workers were working at the scene. One escaped, and another four were rescued by fire fighters but died later in hospital, police said.

The fire broke out on the 13th floor of the Shanghai Telecom Building at 1:32 p.m. The fire was extinguished 20 minutes later.

Shanghai's communication services were not affected by the incident after an emergency response plan was initiated, said local police.