Thousands of homes and buildings were threatened Sunday by an out-of-control wildfire burning in the foothills of a national forest east of Los Angeles, amid a days-long heat wave that pushed temperatures into the triple digits across the region.

State firefighters said 8,733 structures were threatened, including single and multi-family homes and commercial buildings.

The so-called Line Fire was burning along the edge of the San Bernardino National Forest, about 105 kilometers east of LA. As of Sunday morning, the blaze had charred about 70 square kilometers of grass and chaparral, leaving a thick cloud of dark smoke blanketing the area.

County officials declared an emergency Saturday evening.

“Extreme temperatures, wind and lightning strikes have allowed the fire to grow rapidly,” the county said in a statement.

More firefighters were expected to arrive Sunday. State officials said vegetation is critically dry in the area and temperatures reached more than 38 degrees Celsius on Saturday with relative humidity dipping to provide ideal conditions for fire growth.

The fire began Thursday evening, and the cause is under investigation.

About 500 firefighters were battling the blaze, supported by water-dropping helicopters that hovered over homes and hillsides, along with aircraft.

The fire produced coiling clouds of dense smoke, and flames could be seen cresting hillside ridges.

No injuries were reported, and no homes or other structures had been damaged or destroyed.

The National Weather Service said downtown Los Angeles hit a high of 44 degrees Celsius Friday, which marked the third time since 1877 that a temperature that high has been reached there.