LANDUSKY, Mont. — More crews and equipment were brought in Friday to help battle a wildfire south of the Fort Belknap Indian Reservation in north-central Montana as the growing fire threatened the small town of Landusky.

The fire, burning in grass and timber in the Little Rocky Mountains grew from 2.5 square miles (6.5 square kilometers) on Wednesday to 13 square miles (33 square kilometers) late Thursday, after the wind shifted, the Bureau of Land Management said.

The Phillips County sheriff’s office issued a voluntary evacuation order for about 40 residents of Landusky on Thursday, but some people stayed. One outbuilding has burned.

Crews planned to work Friday to protect buildings and strengthen fire lines to prevent further threat to the communities of Zortman and Landusky, the BLM said. Hot, dry weather is expected to continue, with possible thunderstorms that could bring wind and lightning, but little rain.

Three 20-member hotshot crews, several more engines and equipment we being brought in as residents warily watched the rising smoke and flames. Crews worked around the town of Landusky on Thursday, cutting trees and undergrowth to eliminate fuel for the fire, The Billings Gazette reported.

Helicopters dropped buckets of water on hot spots and airplanes were dropping retardant on the hillsides between the town and the Little Rocky Mountains to the north.

One fire crew was stationed in town to extinguish any embers.

“Our goal is to corral this thing and keep it where it needs to be,” said Geremy Olson, press information officer for the Northern Rockies Incident Management Team.

The fire started on Monday afternoon near Zortman and is 10 percent contained. The cause is still under investigation.