CIUDAD JUAREZ, Mexico — With 5 million discarded tires littering the background, the United States and Mexico announced an accord Thursday to clean up the mountains of rusty cars, smashed school buses, and rotting rubber that are a blight on the border.

Environmental officials from both countries said the first step of the massive clean-up would be to start burning the tires for fuel in cement factories.

"The environmental challenges that we face do not respect political boundaries," said Richard Green, a regional administrator for the U.S. Environmental Protection Agency. "We experience them together, and we must address them together.

Mexico's secretary of environment and natural resources, Alberto Cardenas-Jimenez, said Grupo Cementos de Chihuahua and officials of Ciudad Juarez, across the border from El Paso, Texas, have agreed to dispose of 800,000 used tires in each of the next five years.

Ciudad Juarez will pay the cement-maker 31 cents for every tire burned, and in turn, Cementos de Chihuahua will use the tires as fuel, investing $2.5 million in equipment at the Samalayuca cement factory, 25 miles south of Ciudad Juarez.

Though the United States is not involved in funding the project, it worked with Mexican authorities in coming up with the plan. The agreement will only make a dent in the estimated 5 million used tires at the city's collection center. Two million more are scattered in dry gullies and clandestine dumping sites throughout the city, where residents simply throw out tires to avoid paying the $1 fee the city requires to get rid of them.

"These tires have become breeding grounds for disease, carrying mosquitoes, and when they burn, they release hazardous waste to the air and soil making it a grave environmental and health problem for the people of Juarez," said Alma Leticia Figueroa, director of Ciudad Juarez's Ecology and Civil Protection Department.

Figueroa said each month firefighters put out about 1,000 fires started by people often trying to get rid of both trash and tires.

In addition, as part of the accord, tires from Tijuana and Mexicali, also popular dumping grounds, will be sent to nearby cement factories to be used as fuel.

The tires will be burned in ovens that have been approved by Mexican environmental authorities, assuring no pollutants are released in the air, said Gonzalo Bravo, a Mexican organizer of the environmental forum.

"Using tires as fuel in cement factories is common practice in industrialized countries, and for us it has become the best solution," Bravo said.

U.S. and Mexican environmental officials were wrapping up their two-day meeting Thursday with discussions on another major environmental problem: the rapid growth of smog-spewing, beat-up cars that clog the streets of Ciudad Juarez and other major border towns.

In Ciudad Juarez, a city of 1.3 million, only one-fourth of the city's population uses public transportation, and 80 percent of the city's cars are at least a decade old. With the United States so close and also so willing to sell old cars for cheap prices, Juarez and other border towns have become huge markets for junkers. Many don't meet environmental standards.

Along the Mexican border, cars are often driven with missing windows or with the hood held together with rope. When the cars break down, drivers often sell them to junkyards or simply abandon the carcass and buy another one.

Forum participants proposed developing a uniform strategy for inspection and control of used vehicles that would apply to both sides of the border and a Mexican law that would prohibit the entrance of polluting vehicles to the country.

"If we don't have a law that allows me to stop those vehicles, how can we make drivers comply?" Figueroa said. "We want the environmental requirements to be the same on both sides of the border."

In Ciudad Juarez, junkyards have mushroomed in the past six years to about 500 lots holding 2 million cars. An old car often sells for as little as $300, making it accessible to the thousands of workers who earn about $100 a week.

"Used cars in this city are a necessity," said Daniel Obregon, who drives a 1977 Ford pickup truck and works at a junkyard where he makes $90 a week. "Poor people can only afford a used car, and the junkyards are the only way to keep them running."