An early morning fire at a San Francisco Sunset District home spread to neighboring houses on Wednesday, injuring three people and killing a pet cat, authorities said.

The two-alarm fire was reported just after 6 a.m. at a three-story home on 47th Avenue between Irving and Judah Streets. The San Francisco Fire Department said the fire spread to the two adjacent homes on each side, and firefighters were able to quickly extinguish the two secondary fires.

However, the original home on fire presented challenges to firefighters because of its balloon framing, the Fire Department said, where the vertical wall studs run continuously through more than one story, which allows fire to spread up the walls quickly.

Firefighters also rescued three people from the home, one of whom jumped from a backyard window and was taken to a hospital in critical condition, later downgraded to stable condition, the department said. There were four victims identified, three of whom were hospitalized, the department said.

Firefighters also rescued two cats from the home, one of which later died of its injuries.

The intensity of the fire forced firefighters to shift from an offensive interior attack to a defensive strategy to contain and extinguish the fire, the department said.

There was no immediate word on the cause of the fire.