NORTHERN SAN LUIS OBISPO COUNTY, Calif. -

There's been a dramatic spike in the number of homes and buildings destroyed by the Chimney Fire burning in northern San Luis Obispo County near Lake Nacimiento.

CALFIRE says damage assessment teams have identified 30 homes and 10 other buildings destroyed by the fire that's been burning since last Saturday afternoon.

As of Tuesday night, the fire had burned 6,900 acres with containment at 20%.

More than 1,700 firefighting personnel are assigned to help battle the wildfire.

Firefighting crews are working 24-hour shifts on the firelines in triple-digit heat and in extremely dry conditions.

Incident division commanders have assigned fire safety officers to monitor the firefighters and to watch out for so-called "widow makers", blow ups from hot spots of dense, dry fuels and falling trees that are dead or dying from the ongoing drought.

"So trees are just falling over with no warning signs or nothing, just boom", says CALFIRE spokesperson Jay Smith, "that's another concern, so that's why our safety officers are out there just helping us out and making sure they stay safe, making sure we have a proper protection gear on and that we stay hydrated and all that good stuff."

The Chimney Fire started last Saturday afternoon in the area of Chimney Rock and Running Deer Roads and is burning in a northerly direction toward Lake Nacimiento where dozens of homes remain threatened.

"We're talking about triple digit heat, we're talking about relative humidity in the teens, we're getting some good fire activity", Jay Smith says, "we haven't had a burn in this area in the last 60 years, so that just adds up to a fast-moving fire especially when we have winds pushing on it, the fire is moving hot and fast."

Firefighters from the Oregon Border south to San Diego County have been called in to help fight the fire.

So far there have been no reports of firefighter injuries and the cause of the Chimney Fire remains under investigation.