UNIVERSAL CITY – Gov. Arnold Schwarzenegger on Thursday urged fellow governors on both sides of the U.S.-Mexico border to cooperate on issues from water management to building a green economy.

The annual Border Governors Conference spotlights a region that stretches from the Pacific coast to the Gulf of Mexico that is rife with unyielding problems – pollution, gun-running and drug violence – and economic potential.

Illegal immigration has strained relations between the two countries, but Schwarzenegger called the 2,000-mile border “a line that unites us.”

“There is no divide to the air that we all breathe, or the clean water that we all depend on,” said Schwarzenegger, a Republican who chairs the group. “There is no divide between our common desire to make the border region an economic powerhouse.”

The border states – California, Arizona, New Mexico and Texas in the U.S., and Sonora, Baja California, Chihuahua, Coahuila, Nuevo Leon and Tamaulipas in Mexico – plan to sign an agreement Friday to clean up huge discarded tire piles in the region. An agreement is also planned on climate change.

Competing interests at the border make solutions elusive. Officials are eager to promote bustling trade and tourism – Mexico is California's biggest trading partner – but drug trafficking and illegal immigration remain thorny issues.

Federal officials say nearly all illegal guns seized in Mexico are from the United States, and many of the weapons have been traced to smuggling points in Southern California, Arizona, Texas and New Mexico. Near the bustling San Ysidro crossing, assailants in Tijuana throw rocks, bottles and bricks at agents in San Diego, hoping to distract them long enough to jump the fence. The Bush administration is racing to meet a congressional mandate that called for 670 miles of fencing to be in place along the U.S.-Mexico border by the end of the year.

U.S. Homeland Security Secretary Michael Chertoff told the governors that the administration was looking for ways to speed cross-border travel, even as security is tightened in an age of terrorism. Chertoff also said the government wants to increase binational cooperation when floods or wildfires strike.

“We know that when storms or other natural disasters occur, they do not respect political boundaries,” Chertoff said.