Three brush fires broke out in San Diego County Friday, but they were quickly contained as firefighting agencies remained on high alert due to hot and dry conditions.

Near Ramona, a brush fire burned about five acres. It began about 11:50 a.m. at Ash and Alice streets near state Route 78. Ten structures were briefly threatened, but none were damaged or destroyed, Cal Fire spokeswoman Roxanne Provaznik said.

Firefighters expected to have full control of the fire by 6 p.m. The blaze was caused by downed power lines, Provaznik said.

Twelve acres burned in a brush fire that broke out at 12:48 p.m. at Mystery Mountain Road and Old Castle Road near Valley Center, Provaznik said. Structures were briefly threatened, but no buildings were damaged or destroyed.

Firefighters planned to stay at the fire site several hours Friday night to gain full control, Provaznik said. That fire was caused by equipment use, and the investigation is continuing, she said.

Another fire began about 1:45 p.m. near Poway Road and state Route 67. A dispatcher for the San Diego Fire-Rescue Department said the fire consumed about three acres, and no structures were damaged or destroyed. Cal Fire units assisted with firefighters, engines and aircraft.

Provaznik said another fire was burning Friday south of the Imperial County line in Mexico, but as of early evening it had not crossed the U.S. border.

Cal Fire plans to staff an extra five engines over the weekend, raising the number of firefighters almost to peak summer levels. An additional air tanker also will be available at the Ramona air base, said Cal Fire Capt. Nick Schuler.