A major environmental disaster is playing out in southern Mexico, U.S. officials said Friday after assessing forest fires that are sending a choking haze into the United States.

Hundreds of endangered plant species could be destroyed by the blazes in the Chimalapas biological reserve, one of the most important tropical rain forests in the Americas, according to Brian Atwood, head of the Agency for International Development."This disaster in Mexico has to be the most serious of its kind we have seen anywhere in the world, including the fires in Indonesia," Atwood said after traveling Thursday with U.S. Agriculture Secretary Dan Glickman to the part of southern Mexico hardest-hit by the fires.

"American people wonder why we should care about the fires here in Mexico," Atwood said. "It's not just the smoke and haze that's affecting the United States as far north as Wisconsin and as far east as Georgia . . . but it's the potential loss of biodiversity."

About 1,500 of the world's most-endangered species of plants grow in the Chimalapas reserve and 90 percent of the migratory birds that reach the United States stop there, he said. About 130 of the most-used pharmaceuticals in the United States contain ingredients from its forests.

Forest fires in Indonesia have also charred hundreds of thousands of acres since last year, enveloping much of Southeast Asia in smoke.

Atwood said the Mexican fires are especially difficult to fight because the undergrowth is burning while the forest canopies remain intact. That makes it hard to pinpoint the fires, and tough to dump water on them.

In addition, many of the fires are burning in very remote areas accessible only by foot. The two officials praised Mexican firefighters, who are being supported with U.S. equipment and guidance. Sixty Mexicans have died fighting blazes across the country.

"Mexican authorities, in my judgment, have done this as well as we would have done it," Glickman said.

U.S. officials blamed the fires on drought caused by El Nino, and the centuries-old Mexican practice of setting fires to clear land.

Glickman called for an international campaign to train farmers to clear farmland without fires - something that will be difficult in many cultures.

Farmers clearing land start 500 million wildfires every year, Atwood said, causing a fifth of the greenhouse gases in the atmosphere. With the unusual weather of El Nino, the fires spread more easily.

Light rain has begun to fall over much of Mexico, but not yet enough to extinguish the fires, the Americans said. The number of fires peaked last month, but 144 continue to burn, including 23 large fires.

So far this year, 12,627 fires have burned 940,000 acres in Mexico. Fires also continue to burn in Guatemala, Nicaragua, Honduras, El Salvador and Costa Rica.

Atwood's agency has pledged $5 million in aid to help control the blazes in Mexico, and an equal amount for those in Central America.