A wildfire in the US which has killed three people and charred more than 62 square miles could burn all summer even as hundreds of firefighters continue to fight it.

More than 700 firefighters have joined the battle with more expected to arrive to help protect homes and control the blaze.

The fire was 20 per cent contained, and people evacuated in a string of small communities in Utah were told they could return, a spokeswoman for the firefighters said.

Since Friday, when three people were killed, the fire has burned nearly 40,000 acres - about one-third in the Ashley National Forest and the rest on private and public land and the Uintah and Ouray Indian reservations.

The fire is burning about 100 miles east of Salt Lake City.

Fire officials want to let the fire continue to burn in parts of the national forest to clear dead trees.

Some small communities in the area were still under a mandatory evacuation.

In California, a fire that destroyed 13 homes and 18 outbuildings in Kern County was contained on Tuesday and crews were close to ringing a smaller fire near Santa Barbara.