Donald Trump planning to send 800 more troops to Mexico border

US Defence Secretary James Mattis is expected to sign deployment orders that could send over 800 more troops to the border with Mexico to help border patrol authorities stop a migrants' caravan from Central America to enter the US.

Administration officials told CNN on Thursday that final details were being worked out including where the troops will come from and their specific tasks.

It was anticipated that the Pentagon would pay for the bulk of the mission by using active duty troops but it was not yet clear if the Department of Homeland Security would reimburse the Pentagon.

According to the officials, the Pentagon has been under growing pressure from the White House to send troops to the southern border, especially after President Donald Trump repeatedly called for the use of the military.

The troops will not engage in lethal operations to stop the migrants. Instead they are expected to provide fencing, wall materials and other technical support at several key points along the border where it is believed the migrants may try to cross.

They will also provide tents and medical care for border authorities in those areas and retain the right of self-defence, but border patrol officers will still be the ones physically stopping illegal migration, the officials told CNN.

The troops are expected to be in position next week.

On Thursday morning, the caravan left Mapastepec, Mexico, about 80 miles from the Mexico-Guatemala border and it isn't expected to reach the US border for weeks.