DENVER, the United States, June 5 (Xinhua) -- A wildfire burning in southwest Colorado is spreading on Tuesday, threatening many homes as the 2018 fire season just began, local media reported.

As of Tuesday night, the wildfire had grown 8.9 square km to 11.8 square km in 48 hours and was only 10 percent contained.

Mandatory evacuation orders remained in place for more than 800 homes, while another 700 homes were told to be ready to evacuate at any time, according to the Colorado Springs Gazette.

More than 400 firefighters, supported by six helicopters and two air tankers, are fighting the fire, according to Inciweb, the Idaho-based Interagency Fire Agency.

The blaze, dubbed the 416 fire, started north of the city of Durango on private land and in San Juan National Forest, some 20 km north of Durango, Colorado.

The cause of the fire is under investigation.

Fire spokesman Neal Kephart told CBS News that the fire burned down to four-lane Highway 550 Tuesday afternoon, but firefighters have so far prevented it from crossing the highway.

In neighboring New Mexico, a fast-moving fire in Colfax County burned over 145.7 square km and part of the legendary Philmont Scout Ranch.

But thanks to moisture over the weekend, crews had contained 25 percent of the blaze by Tuesday and about 450 people were still battling that fire.