Mexican cartels added a new twist to the drug war this week by threatening to kill U.S. cops who seize their goods.

A U.S. Border Patrol agent drives along the steel wall separating the U.S. from Mexico. (Topping/Getty/Getty Images)

Nogales, Ariz., Police Chief Jeffrey Kirkham said his officers received threats a couple weeks ago after off-duty police busted a pot smuggling ring.

"America is based on freedom. We're not going to be intimidated by the threats, but we are taking them seriously," Kirkham told CNN.

"I've told my officers if they venture into that area off-duty to be armed," he said.

Just which cartel made the threat remains unclear. Violent warnings toward American police are not new, but the Nogales incident marked the first time U.S. officials confirmed a threat.

Kirkham said the turf war between the Sinaloa and Juarez cartels have escalated tensions in the border region.

The police chief told the local newspaper his officers were threatened with sniper fire if they continued to seize drugs when off-duty.

The Santa Cruz County sheriff said there had not been threats against his department.

"They seem to respect an officer who's doing his job," Sheriff Antonio Estrada said of the drug gangs, "but when you do it as a civilian, they really take offense."

Mexican law enforcement endured a deadly week last week as 12 officers were killed in a shootout after an ambush. Federal police also killed 15 gunmen during a shootout at a safe house.

Since the Mexican government declared war on drug gangs in 2006, more than 23,000 people have been killed.