Border Patrol agents are firing tear gas and pepper-spray into Mexico.
It's an attempt to stop assailants hurling rocks, bottles and bricks at them.
But border residents caught in the crossfire are outraged, saying they have been forced to evacuate during some of the counter attacks.
Mexico's acting consul general insists that U.S. authorities stop firing onto Mexican soil. A top Border Patrol official says agents are taking action because Mexican authorities have been slow to respond. He says guards have been attacked nearly 1,000 times during a one-year period.
U.S. officials say the violence indicates that smugglers are growing more desperate as stepped-up security makes it harder to sneak across the border. The assailants try to distract agents long enough to let people dash into the U.S.