An explosion reported in San Francisco's Upper Market neighborhood Tuesday night appears to have been caused by a transformer, according to a fire dispatcher.

Emergency crews responded to an area near Portola Drive and Market Street at around 10:30 p.m.

A PG&E spokeswoman said an outage was reported in the same neighborhood at around 10:15 p.m. The outage was reported at 10:19 p.m. and is impacting 273 customers, she said. She could not immediately confirm the cause of the outage.