* Clinton says unfair to blame Mexico for drug war
* Trucking and trade disputes also on Clinton's agenda
* U.S. Congress worried by police corruption in Mexico
MEXICO CITY, March 25 (Reuters) - An "insatiable" appetite in the United States for illegal drugs is to blame for much of the violence ripping through Mexico, U.S. Secretary of State Hillary Clinton said on Wednesday.
Clinton acknowledged the U.S. role in Mexico's vicious drug war as she arrived in Mexico for a two-day visit where she discussed U.S. plans to ramp up security on the border with President Felipe Calderon.
A surge in drug gang killings to 6,300 last year and fears the violence could seep over the border has put Mexico's drug war high on President Barack Obama's agenda, after years of Mexico feeling that Washington was neglecting a joint problem.
"Our insatiable demand for illegal drugs fuels the drug trade. Our inability to prevent weapons from being illegally smuggled across the border to arm these criminals causes the death of police officers, soldiers and civilians," Clinton told reporters during her flight to Mexico City.
"I feel very strongly we have a co-responsibility."
Clinton said the Obama administration strongly backed Mexico in its fight with the drug cartels and vowed the United States would try to speed up the transfer of drug-fighting equipment promised under a 2007 agreement.
"We will stand shoulder-to-shoulder with you ... Our relationship is far greater than any threat," Clinton said at a news conference in Mexico City.
Crushing the drug cartels, who arm themselves with smuggled U.S. weapons and leave slain rivals, sometimes beheaded, in public streets, has become the biggest test of Calderon's presidency as the bloodshed rattles investors and tourists.
Washington plans to ramp up border security with a $184 million program to add 360 security agents to border posts and step up searches for smuggled drugs, guns and cash.
The Obama administration will spend $725 million to modernize border crossings and provide about $80 million to help Mexico purchase Black Hawk helicopters, Clinton said.
It was unclear whether this would be new money from the United States or whether the Obama administration had already requested the funds from Congress.
In Washington, Senator Joseph Lieberman said Obama's plans were not enough and he would seek $385 million more from Congress to pay for 1,600 more Customs and Border Patrol agents and bolster law enforcement centers in border areas.
"The danger here is clear and present. It threatens to get worse," Lieberman said.
Clinton will use her visit to address a trucking dispute with Mexico and long-running trade and immigration issues.
She said the trading partners were making headway on a spat which saw Mexico slam high tariffs on an estimated $2.4 billion worth of U.S. goods after the U.S. Congress ended a pilot program to let Mexican trucks operate in the United States.
"On the trucking dispute, we are working to try to resolve it. We are making progress," she said, adding that she expects Congress will be receptive to the administration's ideas.
Clinton, whose includes a stop in the northern business city of Monterrey on Thursday, said the thorny issues on the table did not mean that U.S.-Mexico relations were in trouble.
"I don't see it that way," she said. "I think that we have some specific challenges ... but every relationship has challenges in it."
Mexico has felt slighted by a delay in the arrival of drug-fighting equipment pledged by former President George W. Bush, as U.S. officials have sought assurances that the aid would not end up in the hands of corrupt officials or police.
The U.S. Congress this month trimmed the amount of drug aid money it will set aside this fiscal year to $300 million from $400 million last year, under a pledge of $1.4 billion to Mexico and Central America over three years.
Since taking office in Dec. 2006, Calderon has spent more than $6.4 billion on his drug war and sent 45,000 troops and federal police to trouble spots around the country.
Mexico has repeatedly said, however, that its efforts will come to nothing if the United States does not clamp down on the smuggling of U.S. guns used in 90 percent of drug crimes south of the border.
Clinton described the violence Mexico is grappling with as "horrendous" and said cartels were alarmingly well equipped.
"It's not only guns. It's night vision goggles. It's body armor. These criminals are outgunning the law enforcement officials," she said. "When you go into a gun fight, where you are trying to round up bad guys and they have ... military style equipment that is much better than yours, you start out at a disadvantage." (Additional reporting by Catherine Bremer; Editing by Kieran Murray)
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