U.S. Secretary of State Hillary Rodham Clinton on Wednesday pledged to stand "shoulder to shoulder" with Mexico in its violent struggle against drug cartels, and acknowledged the U.S. shares blame because of its demand for drugs and supply of weapons.

She said the United States shares responsibility with Mexico for dealing with violence now spilling across the border and promised cooperation to improve security on both sides.

"The criminals and kingpins spreading violence are trying to corrode the foundations of law, order, friendship and trust between us that support our continent. They will fail," she told Mexican Foreign Relations Secretary Patricia Espinosa. "We will stand shoulder to shoulder with you."

On Tuesday, the Obama administration pledged to send more money, technology and manpower to secure the border in the U.S. Southwest and help Mexico battle the cartels. Clinton also said Wednesday that the White House will seek an additional $80 million to help Mexico buy Blackhawk helicopters.

All that is in addition to a three-year, $1.4 billion Bush administration-era program to support Mexico's efforts. Congress already has approved $700 million. President Barack Obama has said he wants to revamp the initiative.

Obama said Tuesday he wanted the U.S. to do more to prevent guns and cash from illicit drug sales from flowing into Mexico. But Clinton's remarks appeared more forceful in recognizing the U.S. share of the blame. In the past, particularly under the Bush administration, Mexican officials have complained that Washington failed to acknowledge the extent that the U.S. drug demand and weapons smuggling fuels the violence.

"I feel very strongly we have a co-responsibility," Clinton told reporters, adding: "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 deaths of police officers, soldiers and civilians."

Criminals are outgunning law enforcement officials, she said, referring to guns and military-style equipment such as night-vision goggles and body armor that the cartels are smuggling from the U.S.

"Clearly, what we have been doing has not worked and it is unfair for our incapacity ... to be creating a situation where people are holding the Mexican government and people responsible," she said. "That's not right."

Clinton said she would repeat her acknowledgment as loudly and as often as needed during her two-day visit to Mexico City and the northern city of Monterrey. Officials said her priorities included encouraging the administration of Mexican President Felipe Calderon to increase its battle against rampant corruption by promoting police and judicial reform.

Just hours before she arrived, the Mexican army announced it had captured one of the country's most-wanted smugglers, a man accused of controlling the flow of drugs through Monterrey for the powerful Beltran-Leyva cartel.

The measures outlined Tuesday include increasing the number of immigrations and customs agents, drug agents and antigun-trafficking agents operating along the border, as well as sending more U.S. officials to work inside Mexico.

Those measures fall short of calls from some U.S. states that troops be deployed to prevent further spillover of the violence, which has surged since Calderon stepped up his government's battle against the cartels.