(CNN) -- Residents displaced by a giant wildfire near Boulder, Colorado, began returning home Sunday while, 40 miles to the north, another fire forced mandatory evacuations west of Loveland.
About 100 firefighters were responding to a brush fire between Flatiron Reservoir and Pinewood Reservoir, local authorities said. The fire quickly grew from 40 acres to between 600 and 700 acres by Sunday evening, the city said.
"The fire is far from controlled at this point," Merlin Green, division chief of Loveland Fire and Rescue, said Sunday night.
Weather conditions forecast for Monday morning looked favorable in aiding firefighters, he added.
A spokesman with the Loveland Fire Department said there was an unspecified number of evacuations and at least 100 homes may be threatened. At least one structure has been damaged, the city of Loveland said in a statement.
Spokesman Andy Hiller said the city sent evacuation alerts to 1,700 residents and received confirmation of receipt from 511.
Homes in the Pinewood Reservoir and Saddlenotch areas are threatened by the fire, the city of Loveland said.
City officials ordered anyone within a four-mile radius of the affected area to evacuate, and more evacuations are possible as 16 agencies responded.
No injuries have been reported. The cause of the blaze has not been determined, Green said. Rescuers will try to reach horses and cattle still on private property.
Video images showed aircraft dropping bright orange retardants on the blaze. Nine air tankers and four helicopter tankers were deployed to help fight the fire, according to the city of Loveland. More resources and firefighers were arriving late Sunday and Monday morning.
The flames from the fire could be seen for miles.
CNN iReporter Randy Macht of Louisville, east of Boulder, lives on a ridge and said the smoke plume appeared to be growing.
"It looks like it's growing faster than not. But you can never really tell. Grass fires produce a lot of smoke, and forest fires are a different ball game. But it's all in the foothills, and you've got lots of homes and trees," Macht said.
The fire's cause was not immediately known.
Meanwhile, a large wildfire that consumed 160 homes and caused a large evacuation may have been sparked by a fire pit, a law enforcement official told the Denver Post in a report published Sunday. The newspaper did not name the official. Authorities previously said the fire may have started after a vehicle crashed into a propane tank.
The Boulder County Sheriff's Office "is not going to release anything about the cause and origin of the fire until they have the evidence to support their conclusions," county spokeswoman Barb Halpin told CNN.
Residents of about 600 homes were forced to leave last week. Many were allowed into the "burn area" Sunday afternoon during a phased re-entry.
The firefighting team expects to fully contain the Fourmile Canyon fire late Monday or Tuesday. Dry weather is expected, with light winds forecast.
More than 1,000 firefighters from several states have strengthened their lines and are tackling hot spots. The blaze was 73 percent contained Sunday.
The fire, which started Monday morning, has charred more than 6,400 acres of countryside and destroyed 172 structures -- 166 of which were homes -- on Boulder's west side.
CNN's Justin Lear contributed to this report.