REDDING (AP) — The thousands of firefighters making progress against a series of wildfires across California were facing new challenges as strong winds raised concerns that some of the fires could jump containment lines, fire officials said Sunday.

The National Weather Service issued a red flag warning across a wide area of Northern California, with forecasters predicting wind gusts up to 35 miles per hour in some of the fire zones.

The stronger winds come after firefighters were able to improve containment lines during the past few days around the Ponderosa Fire, which is burning about 25 miles southeast of Redding, or about 150 miles north of Sacramento.

"In the past 24 hours the fire has not grown in size, but our main concern today (Sunday) is with winds picking up," said California Department of Forestry and Fire Protection spokesman Daniel Berlant.

"The winds have a lot of potential to spread this fire or to allow it to spot over the perimeter line," Berlant said.

The blaze, sparked by a lightning strike on Aug. 18, was 74 percent contained after about 43 square miles.

The wildfire has destroyed 64 homes and 20 outbuildings, mostly near the tiny community of Manton.

About 300 homes are still considered threatened, but despite the increased winds, officials still expected crews to have the blaze contained Monday.

A second major fire in the region, this one burning in the Plumas National Forest since July 29, has expanded to more than 104 square miles.

The nearly 1,400 firefighters on the lines have the blaze 61 percent contained, but officials also expressed concerns about gusting winds.

"We've got a lot of wind on the fire right now," said fire spokesman Larry Helmerick, as he spoke from a tent that he said was being shaken by the wind.

"We're just hoping to hold it today, because we have today, tomorrow and maybe part of Tuesday where we have this wind-driven stuff," he said.

Firefighters also faced challenges from the wind as they battled a fire burning outside the Mendocino County community of Covelo.

One home and five outbuildings have been burned since the blaze stared Aug. 18, and a fire engine was destroyed by the fire Saturday, said fire spokesman Ralph Gonzales. The four firefighters assigned to the engine escaped injury, Gonzales said.

"It's (the fire) burning in an area that's really dry and hasn't burned for years," Gonzales said.

The blaze was 33 percent contained after consuming more than 41 square miles.