ROCKY MOUNTAIN NATIONAL PARK, Colo.—Strong gusts and low relative humidities have stoked a wildfire burning in Rocky Mountain National Park, prompting the fire's incident commander to order more resources to contain the flames.

The 1,370-acre Fern Lake Fire started Oct. 9. Its cause hasn't been determined. Firefighters have been trying to suppress it, but direct attacks have been limited because the fire is burning in steep, rugged terrain, some of which includes trees killed by beetles.

The fire was active Tuesday due to conditions that were ripe for burning, and sparks from the fire jumped Spruce Creek to the southern side.

The incident commander has ordered more resources, including a helicopter to fight the flames.

The fire is listed at 40 percent contained. The park is open, but trails near the fire are closed.