US president Donald Trump offered on Tuesday to help Mexico hunt the killers of nine Mormon women and children shot dead in a lawless border area – and said the US stood ready to back its southern neighbour in a “war” on the drug cartels.

Gunmen ambushed the members of the LeBaron family – a large clan of Mormons who emigrated to Mexico in the late 19th century – on a rural road on Monday between the states of Sonora and Chihuahua, which border the US.

Three women and six children were killed, including twin babies, said relatives and Mexican officials. Six children were also wounded in the attack, which left the family’s three cars riddled with bullets and badly charred.

“If Mexico needs or requests help in cleaning out these monsters, the United States stands ready, willing & able to get involved and do the job quickly and effectively,” Trump wrote in a tweet.

He praised Mexican President Andres Manuel Lopez Obrador for prioritising the fight against drug trafficking, but said “the cartels have become so large and powerful that you sometimes need an army to defeat an army!

“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.”

Lopez Obrador said he would accept “all necessary cooperation” to get justice for the victims, all of whom lived on the Mexican side but had dual nationality.

But the left-wing populist, who has declared an end to Mexico’s “drug wars”, said he was not looking for a new one.

“We don’t agree [with Trump] on that,” he told a news conference. “War is synonymous with irrationality.”

He later tweeted that he had thanked Trump for his support in a phone call.

The White House said the presidents had spoken about the “growing violent behaviour” of drugs cartels and other gangs.

“President Trump made clear that the US condemns these senseless acts of violence that took the lives of nine American citizens and offered Mexico assistance to ensure the perpetrators face justice,” said spokesman Hogan Gidley.

Devastated members of the LeBaron family took to social media to call for people’s prayers and condemn the lawless violence gripping much of the border region.

“It was a massacre,” Julian LeBaron, an activist who has denounced criminal groups in the area, told Mexican radio network Formula.

He said the victims were on their way to the airport when their SUVs were attacked.

Several children managed to escape and return home on foot, he added.

A three-month-old baby was found wounded but alive beneath her mother’s body.

An eight-year-old girl was also left missing for around 24 hours, before being found several kilometres away, unharmed.

Relatives said the children were all deeply traumatised. Five wounded children were airlifted for medical treatment in Phoenix, Arizona.

Mexican Security Minister Alfonso Durazo said the family may have been mistakenly targeted or caught in the crossfire between rival drug cartels. LeBaron, however, said the family believes they were attacked on purpose.