"The focus for today is to hold what we got," extend the fire lines to contain more of the blaze, and bring in more heavy equipment, said Rich Harvey, incident commander for the fire.

Exhausted firefighters fresh off the front lines described the devastation in some neighborhoods and the challenges of battling such a huge blaze.

"It looks like hell. I would imagine it felt like a nuclear bomb went off. There was fire everywhere. Everything had a square shape to it because it was foundations," said Rich Rexach, who had been working 12-hour days since Tuesday, when flames swept through neighborhoods in this city of more than 400,000 people 60 miles south of Denver.

"Everything you put water on, it was just swallowing it," he said.

President Barack Obama toured the stricken areas Friday after issuing a disaster declaration for Colorado that frees up federal funds. He thanked firefighters and other emergency workers, saying: "The country is grateful for your work. The country's got your back."