A US Forest Service ranger credited with reporting a fire that has blackened nearly 103,000 acres has been charged with starting the blaze.

US federal authorities have arrested a longtime US Forest Service employee for allegedly starting the largest fire in Colorado's history.

Colorado firefighters dug trenches and searched through burned-out areas for hot spots along the southern edge of a 40,800 hectare (102,000-acre) forest fire southwest of Denver.

A wildfire in the Rocky Mountains now covers 100,000 acres and has led to several communities being evacuated.