WASHINGTON (AP) -- Three Mexican police chiefs have requested political asylum in the U.S. as violence escalates in the Mexican drug wars and spills across the U.S. border, a top Homeland Security official told The Associated Press.

In the past few months, the police officials have shown up at the U.S. border, fearing for their lives, according to Jayson Ahern, the deputy commissioner of Customs and Border Protection.

"They're basically abandoned by their police officers or police departments in many cases," Ahern told AP.

Ahern said the Mexican officials - whom he didn't name - are being interviewed and their cases are under review for possible asylum.

In the most recent high-level assassination, a top-ranking official on a local Mexican police force was shot more than 50 times and killed. Drug-related violence killed more than 2,500 people last year alone in Mexico.

"It's almost like a military fight," Ahern said Tuesday. "I don't think that generally the American public has any sense of the level of violence that occurs on the border."

As the cartels fight for territory, this carnage spills over to the U.S., Ahern said - from bullet-ridden people stumbling into U.S. territory, to rounds of ammunition coming across U.S. entry ports.

U.S. humvees retrofitted with steel mesh over the glass windows patrol parts of the border to protect agents against guns shots and large rocks regularly thrown at them. At times agents are pinned down by sniper fire as people try to illegally cross into the U.S.

Mexico's drug cartels have long divided the border, with each controlling key cities. But over the past decade Mexico has arrested or killed many of the gangs' top leaders, creating a power vacuum and throwing lucrative drug routes up for the taking.

President Felipe Calderon, who took office in December 2006, responded by deploying more than 24,000 soldiers and federal police to areas where the government had lost control. Cartels have reacted with unprecedented violence, beheading police and killing soldiers.

In general, violence along the U.S. border has gone up over the years. Seven frontline border agents were killed in 2007, and two so far in 2008. Assaults against officers have also shot up from 335 in fiscal 2001 to 987 in fiscal 2007.

There have been 362 assaults against officers during the first four months of 2008, according to Border Patrol statistics. The pattern has been that when more security resources are deployed along the U.S. border, violence against officers spike in response.

Most assaults are along the San Diego and Calexico, Calif., border, as well as the Arizona border near Yuma and south of Tucson.

Now, about 14,000 U.S. border agents work on the southern border, up from more than 9,000 in 2001.

The Bush administration has requested $500 million to fight drug crime in Mexico. Congress is currently considering the proposal.

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