The US will legally designate Mexican drug cartels as terrorist groups, President Donald Trump has said.

Mr Trump told conservative media figure Bill O'Reilly that he had already been working on the designation for 90 days.

It prompted Mexico's foreign ministry to request an urgent meeting with US State Department officials.

This month, Mr Trump said he would "wage WAR on the drug cartels" after an attack on US citizens in Mexico that killed three women and six children.

The victims, who were Mormons of dual US-Mexican nationality, were killed in an ambush while travelling through a remote area of northern Mexico on 4 November.

Officials said it may have been a case of mistaken identity, but relatives of the victims said the killers must have known whom they were targeting.

The president said on Tuesday: "They will be designated... I have been working on that for the last 90 days. You know, designation is not that easy, you have to go through a process, and we are well into that process."

In a statement released shortly afterwards, Mexico's foreign ministry said: "The foreign minister will establish contact with his counterpart, Michael R Pompeo, in order to discuss this very important issue for the bilateral agenda."

When a group is designated as a terrorist organisation in the US, it becomes illegal for people in the US to to knowingly offer support.

Its members are also banned from entering the US. If they are already in the US, they face being deported.

If financial institutions discover they have funds connected to the group, they are required to block the money and alert the US Treasury Department.