Hundreds of firefighters and first responders are working to contain a rapidly growing brush fire in a mountainous area of southern California that has forced thousands of evacuations.

The Canyon fire erupted in a rural, sparsely populated area of Ventura county on Thursday afternoon and quickly exploded in size, spreading east into Los Angeles county. By 3pm on Friday afternoon, the blaze was 25% contained but covered nearly 5,400 acres (21.9 sq km) , according to authorities.

But fire conditions remain precarious, with temperatures up to 100F and low humidity. Moisture-levels on the brush-covered hillsides are at a record low in the region, and the parched vegetation has fueled the fire’s spread.

“The fire has now crossed into LA county and continues to spread east toward the community of Val Verde,”, according to a 1pm update on Friday from the California Department of Forestry and Fire Protection (Cal Fire).

Firefighters battled through the night to protect homes and other buildings, and to take advantage of lower temperatures. By Friday morning, they were seeing subdued fire behavior according to Andrew Dowd, spokesperson for the Ventura county fire department.

“They were putting it all on the line to bring this fire under control,” he said. One firefighter reported a minor injury, but no civilian injuries were reported. No single-family or multi-family residences were destroyed, but two minor structures were destroyed.

“Air attack and ground crews aggressively attacked the fire to limit its spread and protect residents and structures in the fire’s path,” Los Angeles county officials wrote in an incident update issued on Friday, adding that hard work and operational efficiency stopped widespread damage “even as the high heat and dry conditions pushed the fire’s growth”.

Firefighters are hoping to corral some of the perimeter on Friday, even with scorching daytime temperates that could contribute to extreme fire behavior. The blaze remains a “very dynamic situation”, Dowd said.

“Extreme heat and low humidity in our north county have created dangerous conditions where flames can spread with alarming speed,” the LA county supervisor Kathryn Barger, who represents the district, said in a statement. “If first responders tell you to leave, go – without hesitation.”

The new blaze comes as a huge wildfire in central California became the state’s largest blaze of the year, threatening hundreds of homes and burning out of control in the Los Padres national forest.

The Gifford fire had spread to more than 99,200 acres (400 sq km) by Friday morning and was at 15% containment. It grew out of at least four smaller fires that erupted last Friday along State Route 166, forcing closures in both directions east of Santa Maria, a city of about 110,000 people. It has injured at least four people. The causes of the fires are under investigation.

Wildfire risk will be elevated through the weekend across much of inland California as a heatwave gripping the area intensifies.

But fire activity across California and parts of the American west is also expected to spike in the weeks to come as high temperatures collide with the strong winds that are more prevalent in the autumn months.

“In southern California, the threat is driven by persistent drought, high grass loads, and weakening coastal moisture,” according to a wildfire forecast by the state’s fire department.

Vegetation is already dangerously dry in the state, especially in southern California where thick brush and desiccated shrubs can quickly turn ignitions into infernos, and the coming heat will bake more moisture out of the landscapes.

Fires will be more challenging to control and are expected to exhibit extreme behavior, according to an advisory issued from federal fire analysts this week, which said the mountains and deserts of southern California were seeing “record dry levels”.

The Associated Press contributed reporting