The US official told Reuters on Monday that thousands of American troops could be sent to the southern border but there is no firm figure yet.
Last week, US Defense Secretary Jim Mattis authorized the use of troops and other military resources for stopping the migrant caravan from Central America, Presstv Reported.
Officials at the time said Mattis’ authorization did not include a specific number of troops, something that would be determined at a later point.
The mission aims to bolster President Donald Trump’s efforts to stop Central American migrants trekking toward the United States.
Trump, who campaigned against illegal immigration to win the 2016 US presidential vote, has seized on this caravan in the run-up to the November 6 mid-term congressional elections.
Mattis told reporters traveling with him in Europe on Sunday that the US military has already begun delivering jersey barriers to the southern border.
National Guard troops routinely perform those same functions, so it is not clear why active duty forces are being used.
The National Guard is often used by states to help with border security. But active duty troops are rarely deployed within the United States except for domestic emergencies like hurricanes or floods.
Thousands of migrants from Central America are crossing Mexico toward the United States. Trump threatened he would deploy the military and close the US-Mexico border.
The caravan, which began as a march of a few hundred people from the crime-wracked Honduran city of San Pedro Sula on October 13, swelled into the thousands as it was joined by migrants from El Salvador, Nicaragua and Guatemala.