The fire at the Bosque de la Primavera, "Forest of Springtime," on the edge of the western city of Guadalajara, has consumed 18,500 acres, (7,500 hectares), or about one-quarter of the preserve, officials said Wednesday.

The fire, which began over the weekend, has sent plumes of smoke and ash into Mexico's second-largest city, forcing dozens of schools to close. It is the latest chapter in a larger battle in Mexico to save public forests from development, logging, pollution and fires fueled by droughts.

"It (the fire) was started in a clandestine dump near a squatters' camp where, every year, the inhabitants start fires, clearly deliberately, to take over park land," said Alvaro Garcia Chavez, the chief firefighting official for Jalisco state, where Guadalajara is located.

Hernando Rodolfo Guerrero, the federal attorney general for environmental protection, said Tuesday there was evidence the fire was intentionally set and promised to bring those responsible to justice.

Garcia Chavez said authorities had to extinguish 14 fires in the last year in the same area.

This weekend's blaze got out of control because of unusually dry conditions and the accumulation of leaves and branches on the forest floor, said Environment Secretary Juan Elvira Quesada, though he said the blaze "did not follow the natural pattern of a forest fire."

Elvira Quesada said real estate developers have been trying to build on the edges of the park and he pledged to try to prevent any new construction permits or zoning changes for properties within the park.

Garcia Chavez said that two firefighting teams encountered armed gangs in the woods over the weekend and were forced to temporarily withdraw. They later returned under police guard.

"People with (assault) rifles blocked their path, they didn't say anything, they just made the brigades turn back," he said.

Drug traffickers have set up clandestine labs to turn out synthetic drugs such as methamphetamines around the forest in the past, Garcia Chavez said, "taking advantage of the reserve's protected status to carry out criminal activities."

In 2010, three inspectors for Mexico's federal Environment Department headed into the wooded mountains west of Mexico City to investigate a pollution complaint. Their tortured bodies were found the next day, and authorities say they were killed after they apparently stumbled onto a drug lab.

Elvira Quesada said the Mexican army had joined in efforts to aid and protect firefighters and help coordinate aerial water drops. But, he noted, "the army cannot be in woods every day to guard them."

In many parts of Mexico, illegal loggers have joined or formed violent, well-armed organized crime groups.

Earlier this month, gunmen wielding automatic rifles killed eight residents of the Indian town of Cheran, in neighboring Michoacan state, when the resident went into the hills to check reports of illegal logging.

Residents of Cheran are demanding more protection from loggers, who they say have the backing of La Familia drug cartel.