BAVISPE (REUTERS) - Convoys of vehicles carrying relatives of a group of American women and children slain by unknown gunmen snaked through the dark from as far away as the United States into a remote Mexican region ahead of funerals for the victims to be held on Thursday and Friday (Nov 7 and 8).

Members of breakaway Mormon communities that settled in Mexico decades ago, the three dual-nationality women and six children were ambushed in Sonora state on Monday, leading to US President Donald Trump urging Mexico and the US to "wage war' together on the drug cartels.

Late on Monday, dozens of SUV-style vehicles and pick-up trucks escorted by Mexican National Guard outliers rolled into the municipality of Bavispe, where funerals will be held for two of the women and their families on Thursday.

"We came prepared to sleep on the floor, in tents. Whatever is needed to support the families who died in this terrorist act," said Mr Alex LeBaron, a former Congressman and cousin of one of the women, Ms Rhonita Miller.

The remains of Ms Miller and her children, whose bodies were reduced to ash and bones when the car they were in was shot at and went up in flames, are due to be buried in another village called Colonia LeBaron on Friday.

Mr LeBaron, who was with the convoy, told Mexican radio that mourners had come from the US and across Mexico, bringing food and mattresses for the journey.

The LeBaron family, which came to Mexico in the early 20th century, now claims to be made up of more than 5,000 members.

The authorities and relatives say the killings appeared to be the work of the Juarez and the Sinaloa Cartels, who fight for control of lucrative drug routes that run through the sparsely populated mountainous areas into the US.

Mexico has unleashed its military against cartels since 2006 but despite the arrests or killings of leading traffickers, the campaign has failed to reduce violence. Instead, it has led to more killings as criminal groups fight among themselves.

The victims came from prominent local families, including the LeBarons, Millers and Langfords.

Nestled in the fertile valleys of the Sierra Madre mountains just a few hours' drive south from the US border, the oldest communities stem from the late 1800s, when upheaval over polygamy in the Utah-based church led to their founding.

The settlements have marriage ties to others in the US.