PORTLAND — A wildfire in southeastern Oregon has become the state’s largest blaze in more than a century, charring more than 800 square miles and endangering cattle in a sparsely populated area.

The blaze surpassed the size of a 2002 wildfire that scorched about 780 square miles of the Siskiyou National Forest. It is now the largest Oregon blaze since the mid-1800s, officials said Friday.

It’s one of two large blazes burning near Oregon’s borders with Nevada and Idaho. Another has threatened a handful of homes near Harney Lake, prompting authorities to recommend evacuations.

The larger fire has burned up to the west rim of the Owyhee Canyon and firefighters were trying to keep it from jumping to the other side, to the community of Jordan Valley, said Trish Hogervorst, a spokeswoman.

The blaze has charred rangeland structures such as fences and telephone poles, Hogervorst said, but there are very few homes in the area. The fire is about 50 percent contained, but crews were concerned about forecasts showing so-called dry lighting, which strikes without rain.

“When it hits the ground, everything’s so tinder-dry it automatically starts a new fire,” Hogervorst said.

Although the fire has destroyed a large swath of Malheur County, much of it is already extinguished, said Robin Vora, a fire information officer. It’s charring sagebrush grass that quickly burns through, he said.

“I am deeply concerned with both the immediate impacts of the fire and with the long-term effects on this part of Oregon,” Gov. John Kitzhaber said in a statement.

At 515,000 acres, the blaze is Oregon’s largest in memory, said Brian Ballou, a fire prevention specialist at the Oregon Department of Forestry who has written extensively about the state’s fire history.

A fire in the 1840s ravaged 800,000 acres in the Coast Range, Ballou said. A U.S. Geologic Survey map published around 1900 showed a fire burned 1 million acres in the Cascades in the mid-19th century, but researchers believe the area was more likely charred in separate blazes.

Officials said two families have declined to evacuate from the Harney Lake area threatened by the smaller Miller Homestead fire. Residents in nearby Frenchglen, an unincorporated community of 11 people, were told to be ready to leave.

Frenchglen is best known as a launching point for birdwatchers heading to a nearby wildlife refuge and campers going to Steens Mountain.

Crews brought in three helicopters to help battle the blaze that has spread quickly through grass and sagebrush since igniting on Sunday. The fire was 10 percent contained on Friday morning after burning nearly 160,000 acres.

“There was a little bit of activity in town,” said Ada Takacf, a spokeswoman for the firefighting crew. “There’s a lot of smoke, folks could see the fire line on the crest of the hill … but we had a lot of air resources that were able to get right on it.”

Daytime weather conditions are conducive to fire, but firefighters have been able to make progress at night, Takacf said.

Crews were building fire lines and trying to keep the blaze from crossing State Route 205. The road was briefly closed but had been reopened with a pilot car.

The Red Cross has set up an evacuation shelter at a motel in Burns, about 60 miles north of Frenchglen. Displaced residents parked two campers in the motel parking lot but nobody has required a room, said Tom Farley, director of the Red Cross chapter in Bend.

The agency is prepared to offer food, shelter, medication, eyeglasses, comfort and other support that displaced residents may need, Farley said.

U.S. Rep. Greg Walden, a Republican who represents the affected areas, said the U.S. Department of Agriculture would find farmlands in nearby counties where ranchers can relocate their cattle for emergency grazing.