Flames from a backfire burn around a fire truck battling the Delta Fire in the Shasta-Trinity National Forest, Calif., on Thursday, Sept. 6, 2018. (AP Photo/Noah Berger)
Flames from a backfire burn around a fire truck battling the Delta Fire in the Shasta-Trinity National Forest, Calif., on Thursday, Sept. 6, 2018. (AP Photo/Noah Berger)
Photo: Noah Berger/AP Photo.
Photo: Noah Berger/AP Photo.
Embers fly above a firefighter as he hustles to control a backfire as the Delta Fire burns in the Shasta-Trinity National Forest, Calif., on Thursday, Sept. 6, 2018.  Photo: Noah Berger/AP Photo.
Embers fly above a firefighter as he hustles to control a backfire as the Delta Fire burns in the Shasta-Trinity National Forest, Calif., on Thursday, Sept. 6, 2018. Photo: Noah Berger/AP Photo.
Photo: Noah Berger/AP Photo.
Photo: Noah Berger/AP Photo.

LOS ANGELES - Firefighters battled on Saturday to outflank a wildfire that has forced the closure of an interstate highway in Northern California as the blaze swept through explosively dry mountain timber in the Shasta-Trinity National Forest for a fourth day.

The Delta Fire has scorched more than 31,000 acres (12,545 hectares) in the Cascade range since erupting on Wednesday in a forest canyon along the Sacramento River, about 250 miles (402 km) north of San Francisco, fire officials said.

Although containment of the blaze, a measure of the progress made in carving buffers around the fire's perimeter to halt its spread, remained at zero crews have made gains clearing away tinder-dry brush beyond its leading edge.

The fire has spread quickly through drought-stricken pine forests thick with dead and dying timber which has been ravaged by bark beetle infestations.

The underbrush is being removed with a combination of bulldozers and hand tools, as well as controlled burning operations, Captain Brandon Vaccaro, a spokesman for the Delta fire incident command, said.

"We're actually getting a lot of work done and making a lot of good progress, but that progress is not at the edge of the fire," Vaccaro told Reuters by telephone. "We're removing the fuel from in front of the fire."

Much of the effort has also focused on protecting scattered homes and small communities in the sparsely populated fire zone, but an unspecified number of homes have been destroyed, he said.

Vaccaro said about 300 people were under mandatory evacuation orders in Shasta and Trinity counties. Farther north, an evacuation warning was in effect for the town of Dunsmuir, advising some 1,600 residents to be ready to flee at a moment's notice.

The blaze also has caused major travel disruptions in the region. On Wednesday, flames raced across Interstate 5, chasing a number of truckers from their vehicles before flames engulfed their abandoned rigs, though no serious injuries were reported.

A 45-mile (72-km) stretch of the I-5, a key north-south route through the entire state, has remained closed since then, requiring traffic detours of up to 120 miles (193 km), officials said.

Shasta County communities are still recovering from a devastating blaze this summer that killed eight people and incinerated hundreds of dwellings in and around Redding.

California's wildfire threat could intensify next week with forecasts of a statewide heatwave.