The US says "everything is on the table" to keep migrants out, as it unveiled the first phase of a wall at the Mexico border.
The country's Homeland Security Secretary Kirstjen Nielsen said the administration was looking at all legal options to keep migrants out.
Her comments come as a caravan of migrants made up of thousands fleeing violence and poverty in Central America is making its way through Mexico.
Ms Nielsen was speaking as she stood next to a newly constructed 30ft fence in California, which the administration says is the first phase of the border wall.
Ms Nielsen said: "Let me be clear: Walls work."
#CBPElCentro #USBP leadership met with @SecNielsen this morning to discuss the completion 1st section of the #Calexico border wall. Thanks to the Secretary for all her support in securing this section of border. @DHSgov @CBP @CBP_McAleenan pic.twitter.com/AEuBV4TosS
— CBP El Centro (@CBPElCentro) October 26, 2018
The Pentagon has approved a request for extra troops at the southern border, expected to total at least 800, although the figure could be as many as 1,000.
The White House is looking at new border security measures, including one plan that would use the same mechanism as Donald Trump's travel ban to block migrants from seeking asylum in the US.
Ms Nielsen said: "We are looking at every possible way within the legal construct that we have to make sure that those who don't have the legal right to come to this country do not come in."
In a tweet, Ms Nielsen said: "Border security is national security. We need funding to continue building @POTUS' wall.
"We need personnel to secure the border. And we need laws that work for the American people. I refuse to believe that this is too complex for Congress to solve, they must address this crisis."
Image via @CBPElCentro on Twitter
US President Donald Trump has intensified his focus on immigration as the November 6th midterm elections get closer.
The caravan of migrants is about 1,000 miles away but dwindling in size.
Mr Trump said this week: "I called up the military. We're not letting them in, they ought to go back now because we're not."
Any steps the president takes to curtail the rights of migrants to seek asylum are certain to be met with legal challenges.
Andrea Guerrero, executive director of Alliance San Diego, said it would be "a very drastic action that would have disastrous practical implications for our asylum obligations, for our moral and legal obligations".
She added: "We know that civil rights attorneys are preparing to litigate right now.
"They're preparing to litigate right away. I don't know if Trump cares if he wins the litigation.
"He wants to score political points ... it's all part of the same political ploy to stoke fear for political reasons and build more walls."
Details about the military operation at the border have also been revealed.
The US Defence Department said the mission is authorised from October 30th to December 15th and will operate in border areas of California, Arizona and Texas.
