By Catherine Bremer

MEXICO CITY, Dec 10 (Reuters) - Mexico's year-old war on drug trafficking has weakened key cartels but it cannot crush them unless Washington cuts off their illegal supply of arms, cash and chemicals, Mexico's attorney general said on Monday.

A crackdown by 25,000 soldiers and police has put hundreds of smugglers behind bars and extradited dozens to the United States. But drug-related murders continue unabated and are set to top 2,500 this year, up from 2,100 in 2006, officials say.

"We are doing everything we can to stop drugs crossing to the United States but given this is a transnational business by definition it requires the United States do its part and that essentially means the flow of arms to Mexico," said Attorney General Eduardo Medina Mora.

Mexico is still awaiting a delivery date for $1.4 billion of drug-fighting equipment pledged by the United States, amid signs U.S. lawmakers may try to attach conditions to the aid.

"We have done our part, we hope the United States will do its part," Medina Mora said.

Mexico's cartels are hurting after 13,300 people linked to drug smuggling were arrested this year and 89 were extradited.

The narcotics trade has also been hit by a rash of drug busts in Mexico, including a 23-tonne cocaine shipment.

Yet the cartels are still flush with U.S. guns, grenades and drug money that flow down across the border.

Medina Mora said some $10 billion in drug cash flows south each year, and that U.S. gun stores on the border sell twice as much as outlets elsewhere in the United States, implying a brisk trade to Mexico.

"There's a very large flow of money from the United States to Mexico which has no other explanation than drug trafficking. The U.S. government has a very important job to do," he said.

He said Mexico also needed U.S. authorities to clamp down on the movement of Chinese- and Indian-made chemicals, now banned in Mexico, that are flown in for the manufacture of methamphetamine, an increasingly popular illegal drug.

The United States has acknowledged it needs to attack its consumption of cocaine and its illegal gun trade to Mexico, and recently pledged $1.4 billion to help Mexico's drug war.

But as Congress finalizes the aid package, some lawmakers are calling for Washington to have a say in how the money is spent and in ensuring the crackdown respects human rights.

President Felipe Calderon, who made the drug crackdown his first major move on taking office a year ago, said last week the war would not be won during his six-year term. Media reports say smaller drug gangs are now flourishing in areas where bigger cartels have been weakened. (Editing by Doina Chiacu)

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