IMNAHA, Ore. — About 300 firefighters assisted by air tankers and water-dropping helicopters were building containment lines Sunday to combat a growing wildfire in remote eastern Oregon that has forced evacuations of campers, authorities said.

The Double Creek fire near the community of Imnaha in Hells Canyon Recreation Area grew to about 37 square miles, fed by gusty winds on Saturday, according to the National Interagency Fire Center.

The fire prompted evacuations by campers from Imnaha south to Pallette Ranch, the Wallowa County Sheriff’s Office said. The fire, burning on conifer timber stands and grass and shrubs, was detected Tuesday and started by lightning.

Gov. Kate Brown on Saturday issued a declaration allowing the state fire marshal to take command and send other agency resources to help local firefighters. Crews were focusing Sunday on building containment lines at the fire’s southern edge and along the Imnaha River corridor.

Firefighters were monitoring two other lightning-caused fires in the nearby Eagle Cap Wilderness. The Nebo fire has charred 11 square miles of rugged terrain, while the Sturgill fire had grown to 13 square miles. Crews were allowing the two fires to feed on dry fuels while protecting structures.

In southwest Oregon, officials said Sunday that firefighters had contained more than a quarter of the perimeter of the 30-square-mile Rum Creek fire and expected further progress with cooler weather, allowing commanders to release personnel to work on other fires.

The blaze, caused by lightning Aug. 17, destroyed one home and two other structures.