The Line Fire began on Sept. 5 and has since burned some 20,553 acres east of Los Angeles, with three firefighters so far injured attempting to control the blaze, fire officials said. As of 9:25 p.m. Sunday night local time, the fire remained at 0% containment, the California Department of Forestry and Fire Protection said.
No structures have been damaged or destroyed, but the Cal Fire's Sunday evening update warned that 36,328 structures were under threat.
"Line Fire behavior was moderated Sunday morning due to smoke, but as the afternoon progressed, the smoke cleared and temperatures climbed leading to more fire activity," Cal Fire said.
"Night flight capable aircraft will be utilized when possible to try to hold the fire within [current] containment lines," the update said. "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."
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.
"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 in Los Angeles Country has burned around 800 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 the effort to control the blaze.
The Boyles Fire in California's Lake County was at 90 acres late Sunday, 10% 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.
Fire fighters 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.