The suspects are accused of conspiring to buy firearms, including assault rifles, for Mexico's Sinaloa cartel.

The US is under pressure to curb the flow of guns into Mexico, where more than 15,000 people were killed in drug-related violence last year

A federal grand jury indicted a total of 34 people with conspiring to buy guns and illegally export them to Mexico for use by drug cartels.

The suspects are accused of acting as "straw purchasers" by claiming the weapons they bought from licensed Arizona gun shops were for their own use, when in fact they were destined for the Sinaloa cartel, the prosecutor's office said.

"The massive size of this operation sadly exemplifies the magnitude of the problem - Mexican drug lords go shopping for weapons of war in Arizona," the statement added.

The guns included AK-47 assault rifles, a weapon of choice for drug cartel gunmen.

All of those indicted are US citizens or legal residents.

The indictment came a day after US Secretary of State Hillary Clinton visited to Mexico to reiterate US support for President Felipe Calderon's fight against the cartels.

Mexico has long been pressing the US to do more to stop the flow of guns across the border.

Since President Calderon took office in late 2006, more than 34,000 people have died in drug-related violence in Mexico, the government says.

In the same period, Mexico's police and army have seized more 93,000 guns from alleged drug traffickers. Many of the weapons originate in the US.