MEXICO CITY (AP) — 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.

“It is incomprehensible how the United States can be responsible for the massacres we see in different places,” Salazar said in a news conference in Chihuahua on Saturday. “What 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 Joaquín “El Chapo” Guzmán abducted an elder cartel figure, Ismael “El Mayo” Zambada, 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 Andrés Manuel López Obrador blamed American authorities in part for the bloodshed.

Asked at his morning briefing if the U.S. government was “jointly responsible” for this violence in Sinaloa, the president said, “Yes, of course ... for having carried out this operation.”

“If we are now facing instability and clashes in Sinaloa, it is because (the American government) made that decision,” López Obrador said.

López Obrador claimed American authorities “carried out that operation” to capture Zambada and that “it was totally illegal, and agents from the Department of Justice were waiting for Mr. Mayo.”

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, López Obrador — a populist prone to lashing out at critics — said he was putting relations with the U.S. and Canadian embassies “on pause” after ambassadors criticized his controversial plan to overhaul Mexico’s judiciary by requiring all judges to stand for election.

Still, the Zambada capture has fueled criticisms of López 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 López Obrador, who leaves office at the end of this month, cartels have employed an increasing array of weapons and tactics, including roadside bombs, trenches, home-made armored vehicles and bomb-dropping drones. 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.

“The reality is that there is a problem of insecurity and violence” in Mexico, Salazar said.