The pre-positioning -- part of what President Donald Trump has cast as efforts to counter "dangerous" would-be immigrants moving through Mexico toward the US border -- will be completed two days ahead of midterm elections in which the president has sought to make immigration a key issue.

The figure of more than 7,000 American troops corresponds to what has previously been announced: that 5,239 active duty soldiers will join 2,100 members of the National Guard already deployed, said Michael Kucharek, a spokesman for the US military's Northern Command.

The troops will be placed on main military bases in California, Arizona and Texas, but their final deployment has yet to be determined, Kucharek said.

According to a Pentagon official who spoke on condition of anonymity, a thousand soldiers had already arrived Friday in McAllen, Texas, on the border with Mexico.

Trump has said that as many as 15,000 US military personnel could be sent to the border, but these forces will likely be limited to support activities as they are legally barred from conducting law enforcement operations in the United States.

Trump has nonetheless warned that they could open fire if migrants throw rocks at them.