Drug cartel gunmen ambushed three SUVs along a dirt road, slaughtering at least six children and three women — all of them U.S. citizens living in northern Mexico — in a grisly attack that left one vehicle a burned-out, bullet-riddled hulk, authorities said Tuesday.

The dead included eight-month-old twins. Eight children were found alive after escaping from the vehicles and hiding in the brush, but at least five had bullet wounds or other injuries and were being treated in the U.S., where they were listed as stable, officials and relatives said.

The gunmen apparently killed one woman, Christina Langford Johnson, after she jumped out of her vehicle and waved her hands to show she wasn't a threat, according to an account published by family members and corroborated by prosecutors and a relative in a telephone interview.

She was found 15 metres from her Suburban, shot to death. Her seven-month-old daughter, Faith Marie Johnson, was discovered uninjured in her car seat.

Kendra Miller, a relative, wrote that the baby's car seat "seemed to be put on the floor, by her mother to try and protect her … She gave her life to try and save the rest."

A short distance away, Dawna Ray Langford, 43, lay dead in the front seat of another Suburban, along with the bullet-riddled bodies of her sons, ages two and 11.

Around the ambush scene, which stretched for miles, investigators found more that 200 spent shell casings, mostly from assault rifles.

"Lately it's getting worse. This is a whole new level," Taylor Langford, a relative of the dead who splits his time between the Mexican community and his home in the Salt Lake City suburb of Herriman, Utah, said of drug-related violence.

Security Secretary Alfonso Durazo said the gunmen may have mistaken the group's large SUVs for rival gangs' vehicles. The attack took place in a remote, mountainous area where the Sinaloa cartel has been engaged in a turf war with another gang.

All of the victims were believed to be members of the extended LeBaron family, who live in a decades-old settlement founded by an offshoot of the Utah-based Church of Jesus Christ of Latter-day Saints. Another LeBaron family member was killed a decade ago after denouncing the cartels.

"There's apparently a war right now," a relative who did not want his name used for fear of reprisals said wearily. "It's been going on for too long."

U.S. President Donald Trump said the U.S. was ready to help in the investigation if asked.

In a tweet, he urged Mexico to "wage WAR on the drug cartels and wipe them off the face of the Earth."

This is the time for Mexico, with the help of the United States, to wage WAR on the drug cartels and wipe them off the face of the earth. We merely await a call from your great new president! —@realDonaldTrump

Mexican President Andres Manuel Lopez Obrador said of that approach at Tuesday's news conference, "The worst thing you can have is war."

"We declared war, and it didn't work," Lopez Obrador said, referring to the policies of previous administrations. "That is not an option."

Still, it was the second failure in recent weeks for Lopez Obrador's "hugs not bullets" anti-crime strategy. Two weeks ago, Mexican troops had to release a drug lord after his supporters mounted armed attacks in Culiacan, Sinaloa.

Mexican Security Minister Alfonso Durazo, seen last month, said the gunmen may have mistaken the group's large SUVs for rival gangs. (Reuters)

Mexico's federal Department of Security and Citizens' Protection said late Monday that security forces were reinforced with National Guard, army and state police troops in the area following "the reports about disappearance and aggression against several people." The troops were searching for the missing community members, believed to include 11 children or more.

Romney 'heartbroken'

The victims lived in the hamlet of La Mora in Sonora state, about 110 kilometres south of Douglas, Ariz. A number of extended families from Mormon Church breakaway groups live in farming communities clustered around the Chihuahua-Sonora state border.

Many members were born in Mexico and thus have dual citizenship. While some of the splinter groups were once polygamous, many no longer are.

Some of the children who escaped had grisly wounds. One had been shot in the face, another in the foot. One girl suffered gunshot wounds to her back and foot.

Cowering in the brush, one boy hid the other children and then walked back to La Mora to get help. Another girl, who was initially listed as missing, walked off in another direction, despite her gunshot wounds, to get help.

Anti-crime activist Benjamin LeBaron, left, who was killed in Chihuahua state and was related to those killed in Monday's attack, and his neighbour Luis Widmar, in a photo shown at a 2009 funeral service. (The Associated Press)

A group of male relatives set out to try to rescue the youngsters but turned back when they heard gunfire ahead.

A relative of the dead who did not want his name used for fear of retaliation, said in an interview that when they finally made it to the scene where the ambush started — about 18 kilometres from where the two other mothers were killed — they found a burned-out Chevy Tahoe.

Inside, they saw the charred remains of Rhonita Miller, 30, her 10-year-old daughter, a son, 12, and her eight-month-old twins. They were "burnt to a crisp," the relative said.

The gunmen had riddled the vehicle with dozens of bullets and apparently hit the gas tank, causing it to explode.

"When we were there, the cartels from Sonora, there were probably 50 or 60 of them, armed to the teeth, about a mile on this side," said the relative.

A suspect was detained near Agua Prieta, the Sonora prosecutor's office said, but it was unclear whether the person had taken part in the ambush. The suspect had assault rifles and a .50-calibre sniper rifle and was holding two bound kidnap victims, authorities said.

Utah Sen. Mitt Romney, whose father George was born in a Mormon settlement in Mexico, expressed his sadness over the killings.

"Ann and I are heartbroken for the victims of the horrific attacks in Mexico," he said. "Our prayers are with their families who have suffered such an unspeakable tragedy. The U.S. must work with Mexican officials to hold accountable those responsible for this senseless violence."

Ann and I are heartbroken for the victims of the horrific attacks in Mexico. Our prayers are with their families who have suffered such an unspeakable tragedy. The U.S. must work with Mexican officials to hold accountable those responsible for this senseless violence. —@MittRomney

It is not the first time that members of the breakaway church had been attacked in northern Mexico, where their forebears settled — often in Chihuahua state — decades ago.

In 2009, Benjamin LeBaron, an anti-crime activist who was related to those killed in Monday's attack, was killed in neighbouring Chihuahua state. Gunmen tortured him in front of his family, then killed him and his brother-in-law.

But the 2009 killings — which sparked a further crackdown on cartels — also showed how much worse things have become: No children were killed back then.

In August in Chihuahua state, cartel gunmen fired 123 bullets at a man and also killed three girls, ages 4, 13 and 14.