US President Donald Trump has said he wants to turn American cities into military "training centers".

In an unusual move, he told hundreds of military leaders from around the world that "protests and unrest" are like "the enemy from within".

He added: "If you get involved, it won't get out of hand."

Trump recently deployed National Guard troops to Washington, D.C., Los Angeles, and Portland, Oregon, in an effort to reduce crime and arrest undocumented immigrants.

Defense Secretary Pete Heggesz delivered a speech, announcing the end of the Pentagon's inclusion, social justice, and diversity programs that have been linked to same-sex relationships, gender, and racism, and a new "male-equivalent" physical fitness standard for military officers.

The president reiterated his criticism of Democratic-led cities San Francisco, New York, and Los Angeles, saying they would continue to use the military to enforce law.

"They are places where safety is at a premium; and we will fix them one by one," he said, adding, "That will be a major task for some of you here."