The victims are said to be members of the LeBaron family, linked to a breakaway Mormon community which settled in Mexico several decades ago.

Video of the scene shows a burnt-out car. Some victims may have burned alive and several more people are missing.

Local media say the attack could have been a case of mistaken identity.

LeBaron family members were quoted as saying a group of three mothers and their 14 children had set off in a convoy of cars from Bavispe in Sonora state and were heading to La Mora in the neighbouring state of Chihuahua.

They were ambushed by gunmen in Bavispe. A burnt-out SUV was later found by the side of the road with the remains of the victims.

Family members say those inside the SUV were Rhonita Miller LeBaron and her four children: six-month-old twins, an eight-year-old and a 10-year-old.

Two other women and their 10 children were shot at as they tried to flee.

Some reports say more dead were found in another location, but some of those who fled are believed to have survived.

The Chihuahua state attorney said the number of dead remained "confusing" with family members speaking of nine people killed and local media reporting the death toll could be as high as 12.

Rhonita Miller LeBaron's cousin, Julian LeBaron, called it "a massacre".

The attack is thought to be have been carried out by a drug cartel which operates between the two states.

The governments of Chihuahua and Sonora issued a joint statement saying that an investigation had been launched and additional security forces had been sent to the area.

The victims are members of a community called Colonia LeBaronwhich was founded by a breakaway Mormon group in the first half of the 20th Century after the Church of Jesus Christ of Latter-day Saints in the US starting cracking down on polygamy.

The community now includes both Mormons and Catholics who have settled there. Members are known for standing up to local drug gangs and speaking out about the high levels of cartel violence.

Possible international repercussions

By Will Grant, BBC News, Mexico

Even by the standards of Mexico's brutal drug war this was a particularly horrific attack.

A video reportedly filmed and posted online by a distraught family member shows a completely burnt-out car on a hillside, said to be on the road to La Mora.

The violent ambush is likely to have international repercussions too, as many of those killed were US citizens.

Mexican President Andrés Manuel López Obrador has said he is employing a policy of non-violence and non-confrontation with the country's powerful cartels, but his critics accuse him of lacking any kind of coherent security strategy.

Pressure is growing on the government after an embarrassing episode last month when the police in Sinaloa released the son of the jailed drug lord, El Chapo Guzmán, after they were outgunned by his men.

While local media say the convoy of cars may have been mistaken for that of a rival gang, the LeBaron community has been targeted by the cartels in the past. In 2009, Erick LeBaron was kidnapped for ransom. The community took a stand and said it would not pay for his release as that would just encourage future kidnappings.

Erick LeBaron was eventually released without a ransom being paid. But months later, his brother Benjamin, who had led the campaign for Erick's LeBaron's release, was beaten to death. Benjamin's brother-in-law was also killed.

In 2010, Julian LeBaron published an article in The Dallas Morning News calling for Mexicans to stand up against organised crime.

The Colonia LeBaron community has in the past demanded to be allowed to create its own security force.

This area of northern Mexico is being fought over by two rival gangs, La Línea, which has links to the larger Juárez cartel, and "Los Chapos", which is part of the Sinaloa cartel.

The power and influence of the Sinaloa cartel was on display last month when its members barricaded streets and clashed with security forces in Culiacán after one of their leaders, Ovidio Guzmán López, was arrested.

With the security forces outnumbered and surrounded, the Mexican government took the controversial decision to free Ovidio Guzmán to prevent further bloodshed.