DURANGO, Colo. (AP) — A wildfire burning in southwest Colorado has grown by another 600 acres (243 hectares) but no homes or structures have been lost.

The fire burning about 10 miles (16 kilometers) north of Durango in the San Juan National Forest remains at 10 percent containment. More than 400 firefighters supported by six helicopters and two air tankers are fighting the fire that has burned about 4.6 square miles (about 12 square kilometers).

The cause of the fire, which began last Friday, is under investigation.

Fire spokesman Neal Kephart says the fire on Monday burned down to Highway 550 but firefighters have so far prevented the fire from crossing the highway by aggressively attacking any spot fires that start on the other side of the road.