Firefighters battling the southern part of the largest wildland fire in state history transitioned Thursday to the long task of “mopping up” and making safe vast and still-smoldering burn scars, even as fire crews on the Mendocino Complex fires’ north end struggled to fight the blaze in rugged terrain.

“Mop-up is a huge undertaking that’s relative to the size of the fire,” said Cal Fire Division Chief Todd Derum. “It’s a critically important task necessary to ensure the strength of the control lines.”

Derum said the Ranch fire, the larger of the twin-fire complex, is still burning in its northwest and northeast corners, which fire officials refer to as the fire’s “shoulders.” Terrain in the fire’s northern end falls within the Mendocino National Forest and can be “treacherous” for crews, he said.

The Mendocino Complex fires have burned a total of 305,152 acres and were 52 percent contained. The Ranch fire, at 49 percent containment, had scorched 256,232 acres, while the smaller River fire, at 87 percent containment, had burned 48,920 acres.

Fourteen days after the Mendocino Complex fires ignited, the two blazes have destroyed 119 homes and 110 other structures. Some 9,200 structures remained threatened.

Mendocino and Lake County sheriff’s officials reported that about 540 people — 500 in Lake County and about 40 in Mendocino County — remained evacuated in communities located within the fire perimeter.

“We’ve made a lot of good progress on the fires in the last 24 hours,” said Mendocino National Forest spokesman Will Powers said.

More than 4,000 firefighters were assigned to the fires Thursday, with 404 engines, 93 water tenders, 64 hand crews, 84 bulldozers and 19 helicopters, Cal Fire said.

A red flag warning has been issued by the National Weather Service, spanning through Saturday evening.

“The challenge is that we’ve had basically a really stable weather situation and we’re entering a red flag situation where the temperatures will go up and the humidity will drop, so we might have an increase in fire behavior,” Cal Fire spokesman Ian MacDonald said.

On Thursday afternoon, residents of Spring Valley, High Valley, Landrum Ranch and Double Eagle communities were allowed to return home, the Lake County Sheriff’s Office said in a statement. The mandatory evacuation orders for those communities were reduced to advisory orders, the agency said.

Highway 20 remained open to all traffic.

Mandatory evacuations remained in place beyond road closures in the Upper Lake area, north of the Elk Mountain Road at White Rock Canyon Road and in the Nice and Lucerne areas, at Bartlett Springs north of the Highway 20 intersection, the agency said.

Two firefighters have been injured in the blazes, but Cal Fire did not have further details.

You can reach Staff Writer Martin Espinoza at 707-521-5213 or martin.espinoza@pressdemocrat.com. On Twitter @renofish. You can reach Staff Writer Hannah Beausang at 707-521-5214 or hannah.beausang@pressdemocrat.com.