MEXICO CITY (AP) 鈥 U.S. Ambassador Ken Salazar rejected accusations by Mexico's president that the U.S. was partly responsible for a surge in cartel warfare in northern Sinaloa over the weekend.

Sinaloa has been eclipsed by violence as two warring factions of the Sinaloa cartel have clashed in the state capital of Culiacan in what appears to be a fight for power since two of its leaders were arrested in the U.S. in late July.

鈥淚t is incomprehensible how the United States can be responsible for the massacres we see in different places,鈥 Salazar said in a news conference in Chihuahua on Saturday. 鈥淲hat is being seen in Sinaloa is not the fault of the United States."

The arrests startled many because it appeared that the son of notorious drug kingpin Joaqu铆n 鈥淓l Chapo鈥 Guzm谩n abducted an elder cartel figure, , and flew them both to the U.S. to be detained. Such a violent outburst was expected in the wake of the arrests.

As the warring cartel factions and authorities have clashed in firefights, helicopters regularly circle overhead and military rove the streets of the capital. Families have said they are scared to send their children to school.

Meanwhile, bodies have appeared across the city, often left slung out on the streets or in cars with either sombreros on their heads or pizza slices or boxes pegged onto them with knives. The pizzas and sombreros have become informal symbols for the warring cartel factions, underscoring the brutality of their warfare.

Local authorities said that as of Friday at least 53 people had been killed and 51 others have gone missing in Sinaloa state since the fighting started.

On Thursday, Mexican President Andr茅s Manuel L贸pez Obrador blamed American authorities in part for the bloodshed.

Asked at his morning briefing if the U.S. government was 鈥渏ointly responsible鈥 for this violence in Sinaloa, the president said, 鈥淵es, of course ... for having carried out this operation.鈥

鈥淚f we are now facing instability and clashes in Sinaloa, it is because (the American government) made that decision,鈥 L贸pez Obrador said.

L贸pez Obrador claimed American authorities 鈥渃arried out that operation鈥 to capture Zambada and that 鈥渋t was totally illegal, and agents from the Department of Justice were waiting for Mr. Mayo.鈥

Salazar had previously denied that American officials were involved in the alleged kidnapping.

It was the latest blow to bilateral relations between the two regional allies.

Last month, L贸pez Obrador 鈥 a populist prone to lashing out at critics 鈥 said he after ambassadors criticized his controversial plan to overhaul Mexico鈥檚 judiciary by requiring all judges to stand for election.

Still, the Zambada capture has fueled criticisms of L贸pez Obrador, who has throughout his administration refused to confront the cartels and has falsely stated that cartels respect Mexican citizens and largely fight amongst themselves.

Under L贸pez Obrador, who leaves office at the end of this month, cartels have employed an increasing array of weapons and tactics, including trenches, home-made armored vehicles and . The criminal organizations have also seeped into new industries such as migrant smuggling and the lucrative avocado business.

While Mexican authorities said Saturday they had sent an additional 600 soldiers to Sinaloa as reinforcements, Salazar cast blame for the surge of violence in the state on Mexico's wider security crisis.

鈥淭he reality is that there is a problem of insecurity and violence鈥 in Mexico, Salazar said.

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