A raging wildfire in?the western US state of Colorado, has destroyed at least 135 homes in just three days, authorities said on Wednesday.

Having scorching about 6,200 acres (roughly 160 square km) and forcing the evacuation of about 3,500 people from about 1,000 homes, the fire has become the most destructive in Colorado history, the sheriff's office of Boulder County, Colorado?said.

As of Wednesday, about 10 percent of the fire has been contained, according the office.

It was the first time officials reported being able to hold any part of the 32-km-long fire perimeter.

Four people remain missing as the wildfire swept a canyon in the foothills near Boulder, the office said.

The missing people were among some residents who have stayed behind and risked their lives to try to save their homes, according to the office.

Initially, 20 people were reported missing and 16 have been accounted for, the office said.

Rescue workers continued their search for the remaining four and checked homes in the area, firefighters said.

There have been no reports of deaths or injuries.

Govenor Bill Ritter declared a state of emergency on Tuesday, saying the state was throwing 5 million dollars into the battle to save homes and forests in the Fourmile Canyon area west of Boulder.

Federal emergency authorities have also promised a federal firefighting grant. "It is important right now for people who've been evacuated to just be patient," Ritter said after a survey of the burned area. "This is a very volatile situation."

The cause of the fire is still under investigation.