(CNN) -- A record wildfire raged on in southwestern New Mexico on Sunday, belching out a wall of smoke as it devoured thousands of acres and advanced across the rugged wilderness.
Authorities cautioned children, adults with heart disease and other sensitive groups to stay indoors and avoid the smoke.
The blaze -- the biggest in the state's history -- has scorched an area more than one and a half times bigger than Chicago.
A total of 1,236 personnel are fighting the wildfire, which is 17% contained, U.S. Forest Service officials said Sunday.
Two separate lightning strikes started two wildfires that merged, creating the giant fire.
Lightning ignited the Whitewater Baldy Complex wildfire last month, sparking a blaze that has devoured more than 241,000 acres -- about 380 square miles -- in the southwest portion of the state.
Firefighters made good progress on the northeastern portions Saturday, the forest service said, while along the southwest boundary "reduced fire behavior was observed as the fire moved into gentle sloping hills of pinon and juniper."
On Sunday, firefighters were "scouting the area along Gila Mountain Trail and the Middle Fork River for potential control lines," the forest service said in a statement. A contingency line was being constructed "a safe distance east of the fires along Road 150. Firefighters will also strengthen containment lines south of the community of Mogollon."
An evacuation order in Mogollon will be lifted for residents and business owners will be lifted effective Monday; on Wednesday, the community will be open to the public, the fire service said.
The Baldy Fire started May 9 in an inaccessible area of the rugged wilderness and the Whitewater Fire was reported on May 16 several miles away, according to Forest Service officials.
The two fires in the Gila National Forest merged May 23, enhanced by drought and sustained winds of 40 mph to 50 mph, authorities said.
Extreme drought could mean the smoke in the region will persist until the monsoon season, which typically begins in July, said Catherine Torres, secretary of the New Mexico department of health.
The blaze is 15 miles east of Glenwood, New Mexico.