MEXICO CITY • At least nine members of a Mormon family, all United States citizens, were killed in northern Mexico in an apparent attack by drug cartels, prompting US President Donald Trump to say America would help wipe traffickers "off the face of the earth".

Mexican President Andres Manuel Lopez Obrador declined the offer that Mr Trump made on Twitter, where Mr Trump suggested an army may be needed to fight the gangs.

They later spoke by phone on Tuesday to pledge that justice would be done for the families, said Mexican Foreign Minister Marcelo Ebrard. Prior to the call, Mr Lopez Obrador had rejected what he called any foreign government intervention.

The exchange reflects diverging security policies between the US and Mexico. Mr Lopez Obrador has said he is not changing his security strategy and refuses to counter violence with more violence.

Mr Trump, who has often painted Mexico as a den of crime that threatens the US, is pushing for Mexico to take the opposite tack.

Three women and 14 children were travelling by road when their cars were ambushed. The dead included a pair of eight-month-old twins. Six children survived and two are unaccounted for.

BLOOMBERG, REUTERS