The US government is planning to dispatch 800 or more active duty troops to the southern border as a caravan of thousands of migrants makes its way through Mexico.
The move comes as US president Donald Trump seeks to transform fears about immigration into electoral gains in next month’s mid-term congressional elections.
US defence secretary Jim Mattis is expected to sign an order sending the troops to the border, bolstering US National Guard forces already there.
The action came after Mr Trump spent recent days drawing attention to the caravan of Central Americans slowly making their way on foot into southern Mexico, although still more than 1,000 miles from US soil.
The president and senior White House officials have long believed the issue is key to turning out his ardent base of supporters.
The additional troops would provide logistical and other support to the Border Patrol, US officials said.
It is not unusual for the US National Guard to help with border security. Although active duty troops are sometimes called on for domestic emergencies like hurricanes or floods, they rarely deploy to the southern border.
In the current situation, active duty troops will not be on armed security missions that could lead to a similar incident.
The additional members of the military would assist the Border Patrol by providing things such as vehicles, tents and equipment. There are already about 2,000 National Guard troops there under a previous Pentagon arrangement.
Mr Trump has used the caravan to bolster his election-season warnings that the US is being infiltrated by illegal immigrants “pouring across the border”.
He has claimed without any apparent basis in fact that “Middle Easterners” are among the group.
At rallies and on Twitter, Mr Trump has tried to portray the Democrats as pro-illegal immigration, even claiming, with no evidence, that Democrats had organised and paid for the caravan.
He tweeted that “Democrat inspired laws make it tough for us to stop people at the border” and said he was using the military to respond to what he called a “National Emergency”.
The migrants in the sprawling caravan – once estimated by the United Nations to number more than 7,000 – are hoping to make it to the United States. Most are Hondurans, seeking to escape the poverty and violence that plagues the region, although others have come from Guatemala and El Salvador.
Mr Trump tweeted a direct message to the migrants on Thursday, urging them to return home.
“To those in the Caravan, turnaround,” he wrote. “We are not letting people into the United States illegally. Go back to your Country and if you want, apply for citizenship like millions of others are doing!”