Washington DC - President Donald Trump ordered 1,500 more military personnel to the border with Mexico as part of a flurry of steps to deter immigration, his spokeswoman said on Wednesday.

The president declared a national emergency at the US frontier with Mexico on his first day in office, and the additional personnel will bring the total number of active-duty troops deployed there to around 4,000.

"President Trump signed an executive order for 1,500 additional troops for the United States' southern border," Press Secretary Karoline Leavitt told reporters at the White House.

The additional troops include 1,000 Army personnel and 500 Marines, a senior US military official told journalists at the Pentagon.

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The Marines were on call to support operations to counter devastating wildfires in California but were not activated for that effort, so they have been moved to the border mission instead, the official said on condition of anonymity.

"These forces will work on the placement of physical barriers and other border missions. First operations for them should commence within the next 24 to 48 hours; they're moving right now," the senior military official said.

"We also anticipate there could be some additional airborne intelligence, surveillance, and support assets that would move down to the border to increase situational awareness," the official added.

A senior defense official told journalists the deployment of the additional troops "is the initial effort that we can do right away," saying that "we anticipate many additional missions after this."

Acting Defense Secretary Robert Salesses said the US military would also assist with flights to deport migrants out of the country.

"The department will provide military airlift to support DHS [Department of Homeland Security] deportation flights of more than five thousand illegal aliens from the San Diego, California, and El Paso, Texas sectors," Salesses said in a statement.

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"DHS will provide inflight law enforcement, and the State Department will obtain the requisite diplomatic clearances and provide host-nation notification," he added.

The senior military official said the deportation effort would involve some 100 US military personnel.