The wildfire tearing through the foothills of Southern California's San Bernardino County is now endangering more than 36,000 structures, according to emergency officials.
The Line Fire began on Sept. 5 and has since burned about 21,203 acres east of Los Angeles, with three firefighters injured attempting to control the blaze, fire officials said. As of Monday afternoon, the fire was 3% contained, according to the California Department of Forestry and Fire Protection.
No structures have been damaged or destroyed, but Cal Fire said Monday that 36,328 structures, including homes and commercial buildings, are being threatened by the fire.
"The fire made short, intense runs in the slopes and the lighter fuels which allowed for growth," Cal Fire said in a statement Monday. "Vegetation remains critically dry. Hot and dry weather will continue Monday with an Excessive Heat Warning for the lower elevations of the fire."
Cal Fire officials said they plan to use night-flight capable aircraft when possible "to try to hold the fire within [current] containment lines."
"Early next week, cooler weather will moderate fire activity below the marine layer. However, fire activity above the marine layer will remain active. There is still potential for spot fires to become established well outside the control lines," Cal Fire said in a statement.
Gov. Gavin Newsom declared a state of emergency for San Bernardino County on Saturday, and mandatory evacuation orders have been issued for residents in the path of the blaze. The evacuations were expanded on Sunday.
Newsom said the state's request for Federal Emergency Management Agency assistance was approved by President Joe Biden on Saturday afternoon.
"I thank President Biden for his swift approval of support for the work of our firefighters and first responders battling this fire and protecting local communities," Newsom said.
MORE: Extreme heat, bone-dry vegetation and human misconduct prompting intense wildfire season"It's critical that residents in the impacted areas remain vigilant and prepare to evacuate immediately if called for by local authorities."
The cause of the fire remains under investigation.
Elsewhere, the Bridge Fire, which started on Sunday afternoon in Los Angeles Country, has burned 1,255 acres north of Azusa in the Angeles National Forest, fire officials said. The fire is at 0% containment and evacuation orders are in place for a mobile home park, campgrounds and a small river community. Night-flying helicopters have been deployed in an effort to control the blaze.
The Boyles Fire in Northern California's Lake County, which also started Sunday afternoon, had burned 76 acres by Monday and was 30% contained with Cal Fire noting that "crews are making good progress," though "forward progress has not been stopped." Around 30 structures and between 40 and 50 vehicles were destroyed.
Firefighters are also battling the Davis Fire in Nevada's Washoe County, where 6,500 acres have been burned and the fire is at 0% containment. The fire is "burning in heavy timber and brush," according to the U.S. Forest Service. "Gusty conditions" are forecast through the week, the service added on social media, noting that at least 14 structures were so far impacted.
"Power outages continue," the Forest Service said, with a "portion of south Reno under evacuation notice," impacting between 12,000 and 14,000 people.
ABC News' Marilyn Heck contributed to this report.