Dec 12, 6:02 PM EST

TUCSON, Ariz. (AP) -- Assaults on U.S. Border Patrol agents, including rock throwing and incidents using flammable liquids, are increasing in southwestern Arizona, a hot spot for illegal border crossings, authorities say.

Agency spokesmen in Arizona and Washington say it's a reflection of frustration by smugglers and illegal immigrants who have been unable to make it across without getting caught.

The Border Patrol's Yuma sector, which covers the extreme southwest corner of Arizona, has seen the most significant jump in recent months. But similar spikes in border assaults have occurred previously in other areas, including the patrol's Tucson and San Diego sectors.

From the start of the current fiscal year on Oct. 1 until Friday, there were 57 rock-throwing attacks targeting agents in the Border Patrol's Yuma sector, compared to 36 for the same period in 2005, spokesman Lloyd Easterling said.

"It's a money issue. As we do better, as we tighten up enforcement along the border, it certainly impacts the money the smugglers are making," he said. "Frustration - that's exactly what it is. They're trying to clear us out of the way so they can get back to business as usual. But that's not going to happen."

Arizona is the busiest point for illegal entries from Mexico, and agents and resources added to counter the traffic have been pushing migrants into more remote desert areas, including areas around Yuma.

Last Thursday, a Border Patrol agent was attacked with baseball-sized rocks as he struggled with an illegal immigrant near a border fence in the community of San Luis. He needed a dozen stitches for a head wound.

Last month, a Molotov cocktail thrown across the fence burst harmlessly into flames. So did another bottle filled with flammable material thrown onto a street in the border community of Nogales last year.

Todd Fraser, a Border Patrol spokesman in Washington, said some attackers have used rocks that have been wrapped in gasoline-soaked rags and then set on fire.

The Border Patrol has outfitted many vehicles with protective steel cages or metal louvres over windshields to protect agents from the attacks. Agents also are armed with pepper spray for up-close use, collapsible steel batons and pepperball launchers, and can wear Kevlar helmets if needed.

The head of the union representing Border Patrol agents discounted the Border Patrol's conclusion that increasing attacks are a sign of the agency's effectiveness. T.J. Bonner said it's just one more tactic used by smugglers.

Placing agents right on the border makes it "predictable and easy to ambush them and assault them with rocks," said Bonner, president of the National Border Patrol Council.

"Most of these (attacks) are done for the purpose of trying to further entry; they're a diversion to get the agents to back away so they (smugglers) can get the traffic through."

The trend predates the latest spike in attacks seen in Yuma, he said.

"Two years ago there was about a 108 percent increase nationwide," Bonner said. "And this year seems to be on track to be another year that has a lot of assaults on Border Patrol agents."

Miguel Escobar Valdez, Mexico's consul in Yuma, declined to offer "theories or speculation," deferring to the Border Patrol.

"According to what I have heard from them, as the surveillance has become more intense, this has been one of the consequences," he said.

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