The White House may grant US troops on the border with Mexico the authority to defend customs and border personnel if necessary, an official has said.
The official, speaking to Reuters on condition of anonymity, said a public announcement could come by the end of Tuesday but that the finer details are still being worked on.
There are around 5,900 active-duty troops on the border with Mexico after President Donald Trump sent reinforcements to tighten security before caravans of Central American migrants arrive.
The move came right before US congressional elections, leading critics to claim it was aimed at shoring up the Republican vote.
In a statement, the Department of Homeland Security (DHS) would not comment on whether powers given to the troops would change. But it said it would not allow its officials to be harmed.
"We will do everything we can to protect those who defend our nation's sovereignty and secure our border.
"We appreciate the Department of Defense stepping in to assist the Department of Homeland Security as needed," Katie Waldman, a DHS spokeswoman, said.
The news was first reported by CNN.
About 6,000 Central Americans have reached the border cities of Tijuana and Mexicali, according to local officials.
More groups of migrants are making their way toward Tijuana, with around 10,000 expected.
Additional reporting by Reuters.
