Firefighters worked feverishly Saturday to build containment lines around a fast-moving wildfire that would threaten hundreds of residents if it reached a ridge two miles away.

If the 5,500-acre blaze reaches the ridge, it could get hotter and faster with nothing but dead pine trees in its way, said Debbie Maneely, a fire information officer.

Officials warned residents in Crown King, about 50 miles north of Phoenix, to be ready for an evacuation. No containment figures were available, but crews made "a lot of progress," said Stuart Bishop, a spokesman for the team fighting the blaze.

The North Rim of the Grand Canyon was expected to reopen on Monday after a 58,630-acre blaze forced its closure last week. Two hundred employees who had evacuated Wednesday were returning to prepare for the opening, park spokeswoman Leah McGinnis said Friday.

In Nevada, a 21,120-acre fire continued to burn out of control in the Desert National Wildlife Refuge about 50 miles north of Las Vegas.

The blaze was 30 percent contained, fire information officer Kirsten Cannon said. The area is one of the largest intact blocks of desert bighorn sheep habitat in the Southwest.

The refuge was closed Saturday after several smaller fires sparked by a lightning storm had grown to about 1,000 acres. Those fires, clustered around Gass Peak four miles north of Las Vegas, were visible from the city, Cannon said.

Two lanes of southbound Interstate 15 in Southern California were closed Saturday as firefighters battled a 150-acre wildfire, slowing traffic on the long holiday weekend. The freeway serves as a major artery between San Diego, the inland cities east of Los Angeles and Las Vegas.

Two firefighters suffered heat exhaustion and were taken to a hospital.