La Plata County sheriff’s deputies began knocking on doors in an expanded evacuation area north of Durango on Thursday after the 416 wildfire spread to 5,103 acres in southwest Colorado.

An additional 497 homes were evacuated Thursday morning, bringing the total number to 1,276, said Megan Graham, La Plata County spokeswoman. Pre-evacuation orders have been issued to 750 other homeowners, Graham said.

The wildfire is only 10 percent contained, she said. Firefighting conditions are terrible because the wildfire is burning in steep terrain in a drought area.

“It’s very hot and windy,” Graham said.

A house-by-house mandatory evacuation began at 6 a.m. for homes in the Hermosa area north, from Cometti and Mead Lane to the intersection of La Plata County Road 250C and U.S. 550.

US Highway 550 north will remain closed Thursday morning until evacuations from #416Fire are complete pic.twitter.com/EVpMzgl8Af — La Plata County, CO (@LaPlataCountyCO) June 7, 2018

There are 617 firefighters and 20 aircraft assigned to the fire.

A stretch of U.S. 550 was closed early Thursday until evacuations were completed. The highway reopened from noon until 6 p.m. then was shut down overnight, fire officials said. The busy highway and main traffic artery is scheduled to reopen from 8 a.m. to 6 p.m. Friday, with law enforcement pilot vehicles escorting drivers through the area. The limited opening is subject to change at any moment because of fire behavior.

[ Related: Map of wildfires in Colorado and the United States ]

The smoky fire has fouled the air in the area, an air quality health advisory was posted until at least 9 a.m. Friday, according to the Colorado Department of Public Health & Environment.

The fire is burning north of the Durango city limits. The Durango Airport, which is about 16 miles south of the city, was operating Thursday at full capacity and with no delays, airport officials said.

Officials expect the fire to bear down on the town of Hermosa on Friday, when it could threaten homes, officials said.

“That’s pretty much where we’re putting everyone, most of the crews,” Vickie Russo, a spokeswoman for the team managing the blaze, told the Durango Herald . “The fire is just getting too close for comfort to those structures and the residences.”

No structures have burned so far.

“It is better to give people a window for evacuations instead of, ‘You need to leave right now because the fire is in your backyard,'” Russo said. “We’re still in those severe fire conditions right now, and that’s what we’re up against. Conditions are not in our favor,” Russo said.

The Escalante Middle School at 141 Baker Lane in Durango has been designated as an evacuation center and shelter.

Evacuated residents are asked to check in at the evacuation center to receive credentials for re-entry when conditions allow the evacuation order to be lifted.

Livestock can be evacuated to the La Plata County Fairgrounds at 2500 Main Ave. The emergency call center at 970-385-8700 can help coordinate livestock transport. Small animals are being accepted at the La Plata County Humane Society, 1111 S. Camino del Rio.

The Associated Press contributed to this report.