A factory fire caused flames to erupt in a neighboring residential building in northwest China's Xinjiang Uygur Autonomous Region Wednesday afternoon, and casualties are still unknown, said firefighters.

A plastic factory in Xinjiang's capital city Urumqi caught fire at around 3:30 p.m. and the fire spread to an adjacent residential building, said a firefighter with the municipal fire brigade.

Black smoke spiraled from the six-story building. The building's windows shattered and fell into the yard of the factory.

The fire was under control at 5 p.m., and firefighters are searching the building for possible victims, firefighters said.

Witnesses said that the building's residents were quickly evacuated, but it is unknown whether there are any casualties. An ambulance is being kept on standby.

The cause of the fire is under investigation.