A bushfire burning just outside Boulder, Colorado, has led hundreds of residents to evacuate and has put more than 800 homes on notice that they might have to leave soon as authorities battled the fire amid drought conditions.

There are no reports of significant property damage or injuries due to the fire near Sunshine Canyon, west of the city, said Gabi Boerkircher, a spokeswoman for the Boulder Office of Emergency Management.

But the fire - which had spread to a little more than 25ha - was about 20 per cent contained. The high temperature today was forecast to reach 25C, with possible high winds.

"Not a great day to be fighting a fire, that's for sure," Boerkircher said. She said that 285 homes had received an order to evacuate and 836 had been put on notice to be ready to go if conditions worsen.

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Evacuation calls to residents went out in the middle of the night. Sam and Coila Maphis slept through their 2 am call - a pre-evacuation notice telling them to get ready to go - but were roused when a neighbour began banging on their back window at 4.30 am They packed financial documents, passports and a jewellery box into their car and waited. In the dark, pre-dawn hours, they could clearly see the fire burning; they said it looked like a sunset sitting on top of a dark ridge.

"It was very, very vivid. You could really see the crimson, orange and red. And then it came over the ridge, started to walk down the hill," said Sam Maphis, a retired engineer. "That's when we started to get worried."

The response to the fire included eight aircraft, which were dropping a fire retardant called slurry on the area, according to the Boulder Office of Emergency Management. The cause of the fire is not yet known, Boerkircher said.

Eastern Colorado has been experiencing drought conditions for the past three to four weeks.