By Tim Gaynor

PHOENIX, March 18 (Reuters) - An Arizona judge threw out criminal charges on Wednesday against a gun dealer accused of knowingly selling weapons to smugglers shopping for Mexican drug cartels, after he ruled the prosecutor's evidence was flawed.

Maricopa County Superior Court Judge Robert Gottsfield issued a directed verdict of not guilty in the trial of George Iknadosian, 47, the owner of X-Caliber guns in Phoenix.

Prosecutors alleged Iknadosian sold hundreds of AK-47 assault rifles and other guns to third-party buyers for the cartels in Mexico, where 6,000 people were killed in drug violence last year.

Curbing spiraling drug cartel violence is a top concern for authorities in the United States and Mexico, where nine out of 10 guns recovered from crime scenes are traced back to U.S. gun dealers.

The case was the most significant to date brought by U.S. prosecutors seeking to curb the illegal flow of arms south of the border.

Gottsfield dismissed the case on the grounds that the prosecutors had not proven the third-party buyers, or "straw purchasers," had misrepresented their identities when buying the guns.

"There is no proof whatsoever that any prohibited possessor ended up with the firearms," he said in a ruling.

Since 2006, Mexico has sent tens of thousands of troops to fight powerful cocaine cartels locked in a bloody war for control of lucrative cross-border smuggling routes to the United States.

U.S. Senate lawmakers are to hold hearings in coming weeks to assess the ability of U.S. security forces to handle the rise in crime on the U.S. side of the border related to Mexican traffickers.

Mexican authorities have ordered thousands of additional troops to restore order in Ciudad Juarez -- just south of El Paso, Texas -- where more than 2,000 people have been killed since the start of last year. (Reporting by Tim Gaynor; Editing by Peter Cooney)

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