Since taking office a year ago, Mexican President Andres Manuel Lopez Obrador has stressed a policy of non-intervention, both in other countries' affairs and in Mexico's by other countries.

He has also touted good relations with President Donald Trump while ignoring Trump's barbs toward Mexico.

But the recent killing of nine dual citizens in northern Mexico may make Mexican security issues a target for Trump as he pursues reelection.

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MEXICO CITY (Reuters) - The massacre of nine American citizens has dealt a new blow to the Mexican president's aim to free his country of foreign interference, enveloping him in a growing security crisis just as US counterpart Donald Trump seeks reelection.

On Monday, gunmen killed three women and six children from a Mexican-American Mormon family in northern Mexico, prompting Trump to offer President Andres Manuel Lopez Obrador help in wiping out drug gangs he blamed for the ambush.

The incident is still under investigation, but Trump's words revived emotive warnings about the country he made during his 2016 tilt for presidency, when he accused Mexico of sending "bad hombres" across the US border.

A history buff who makes frequent references to past US and French invasions of Mexico, Lopez Obrador is pursuing a nationalist economic vision, and is staunchly committed to a policy of non-intervention in other countries' affairs.

But in his dealings with the United States, he has repeatedly been obliged to bend policies to satisfy Trump.

Five months ago, Trump responded to a surge in illegal immigration to the United States by threatening Mexico with tariffs if it did not do more to contain the problem.

Lopez Obrador duly obliged with a deployment of security forces the US president credits for a sharp drop in apprehensions of illegal immigrants at the US border.

After the migrant episode, the killings will give Trump more leverage on Mexico as he gears up for reelection, said Gustavo Madero, an opposition senator for Chihuahua.

"The pathway has been opened to use this on drugs now, not just migrants, so that Mexico can be put in a bind as soon as Donald Trump wants to impose other conditions," Madero told Reuters. "These are the arguments he's been waiting for."

Relatives of slain members of Mexican-American families belonging to Mormon communities next to the burnt wreckage of a vehicle where some of their relatives died, in Bavispe, Sonora state, November 5, 2019. Reuters

The massacre of the women and children, who belonged to families from breakaway Mormon communities established decades ago in the northern states of Sonora and Chihuahua, was the latest in a spate of mass killings to roil Mexico.

Homicides were at unprecedented levels when the leftist Lopez Obrador took office in December, pledging to curb violence.

Instead, murders have risen, and the tally in 2019 is on track to surpass last year's record of more than 29,000.

A shocking series of slayings has increased concern at home and abroad over the president's strategy, which has been built on addressing the root causes of poverty and crime, and the creation of a new militarized police force, or National Guard.

That approach came under sustained attack last month when gun battles broke out in center of the northern city of Culiacan during an abortive attempt by security forces to capture a son of imprisoned kingpin Joaquin "El Chapo" Guzman.

'Failed state'

National Guard troops keep watch while escorting a caravan of vehicles with relatives and friends arriving for the funerals of slain members of the Mexican-American Mormon families, in Bavispe, Sonora state, November 6, 2019. Reuters

The morning after the killings, Trump on Twitter urged Lopez Obrador to call him so the two could jointly pursue those responsible for the killings in Sonora.

When informed about Trump's tweets, Lopez Obrador told a news briefing he believed Mexico could handle the case without any intervention by a "foreign government."

"It's a matter for us deal with, the Mexican government, independently, and upholding our sovereignty," he said.

But he quickly called Trump and thanked him for his offer of help — while repeating his mantra that the problem of violence in Mexico could not be met with more violence.

By then, some Republican US Senators were already calling on Lopez Obrador to pay more heed to the United States.

"The hard truth is that Mexico is dangerously close to being a failed state," Sen. Ben Sasse of Nebraska said in a statement.

"Mexico's president hasn't taken the threat seriously and innocent American lives have been lost. The Mexican government must partner with US intelligence and law enforcement agencies to wage a full-scale offensive against these butchers," Sasse said.

Mexican President Andres Manuel Lopez Obrador at his daily news conference, in Oaxaca, October 18, 2019. Reuters

Trump has adopted a friendlier tone with Lopez Obrador than with his predecessor, Enrique Peña Nieto. However, the pitch for joint security cooperation echoed comments he made to Pena Nieto during a tense phase in relations in 2017.

Lopez Obrador likes to stress how good his ties with Trump are, and he has repeatedly declined to reply to the US president's more provocative remarks about Mexico.

But the Mexican president has sought to encourage US lawmakers to ratify a new Trump-brokered North American free trade deal that has already passed in Mexico, which depends on US buyers for around 80% of its exports.

Democrats concerned about US job losses to Mexico, and wary of handing Trump a victory, have so far held up approval of the accord, the United States-Mexico-Canada Agreement (USMCA).

The slaughter of nine US citizens in Mexico is unlikely to help persuade them that USMCA ratification is urgent, said Sergio Alcocer, a former Mexican deputy foreign minister.

And if Trump emerges from impeachment hearings to compete in the 2020 election, Mexico's security problems risk giving him another pretext to go after its government, Alcocer added.

"I think he's holding it in reserve," he said.

(Reporting by Dave Graham; editing by Jonathan Oatis)