(Adds background, attempt to reach Mexico's government)

WASHINGTON, Nov 26 (Reuters) - U.S. President Donald Trump said in an interview aired on Tuesday that he will designate Mexican drug cartels as terrorists over their role in drug and human trafficking.

"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," Trump said in an interview with conservative media personality Bill O'Reilly that aired on Tuesday.

Once a group is designated as a terrorist organization, under U.S. law it is illegal for people in the United States to knowingly offer support and its members cannot enter the country and may be deported. Financial institutions that become aware they have funds connected to the group must block the money and alert the U.S. Treasury Department.

Mexico's government did not immediately respond to requests for comment.

Mexico's foreign minister, Marcelo Ebrard, said on Monday he did not expect the United States to make such a move.

Earlier this month, Trump, in a tweet, offered to help Mexico "wage WAR on the drug cartels and wipe them off the face of the earth."

The proposal came after nine Americans were killed in an ambush in northern Mexico.

Mexican authorities said they may have been victims of mistaken identity amid confrontations among drug gangs in the area.

But the LeBaron extended family, members of a breakaway Mormon community that settled in northern Mexico decades ago, has often been in conflict with drug traffickers in Chihuahua and victims' relatives said the killers must have known who they were targeting. (Reporting by Makini Brice and Eric Beech; additional reporting by Diego Ore and Ana Isabel Martinez in Mexico City; editing by Chris Reese and Richard Pullin)

Our Standards: The Thomson Reuters Trust Principles.