President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE said Tuesday he wants to deploy U.S. troops to guard the southern border until his proposed wall is built, a move that could significantly escalate his efforts to crack down on illegal immigration.

The president told reporters that he has been discussing the idea with Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE, arguably his most trusted Cabinet adviser.

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“Until we can have a wall and proper security, we are going to be guarding our border with the military,” Trump said during a meeting with Baltic state leaders, with Mattis sitting at his side. “That's a big step. We really haven't done that before, or certainly not very much before.”

For the past three days, Trump has vented his frustration about the lack of progress in building a wall along the U.S.-Mexico border, one of his signature campaign promises, along with what he sees as “weak laws” against illegal immigration.

The president’s latest proposal comes as he faces mounting pressure from his base to secure a policy win on immigration after lawmakers ignored his plea for $25 billion in wall money.

Trump received just $1.6 billion for border fortifications in a recent government funding bill, and most of that money cannot be used to build new portions of the wall.

The president has previously suggested that the Pentagon could redirect funding to pay for the wall, but such a move would require congressional approval that the president would likely not receive.

The White House said in a Tuesday evening statement that Trump and his team discussed “the mobilization of the National Guard” as part of a new border strategy, but provided no further details.

The Pentagon did not respond to a request for comment. The Department of Homeland Security, which is tasked with guarding the border, declined to comment.

Sending troops to the border would be an unusual but not unprecedented step.

Former President George W. Bush deployed 6,000 National Guard troops to the southern border in 2006 to assist the Border Patrol while more border agents were hired and trained. Former President Obama made a similar deployment in 2010, sending 1,200 guardsmen to the U.S.-Mexico frontier.

Active-duty personnel face strict limits on their ability to operate on U.S. soil. An 1878 law bans federal troops from enforcing domestic laws, a concern that critics raised with Obama. National Guard troops primarily served in support roles during past border deployments.

Republican and Democratic lawmakers said Trump’s proposal could inflame tensions with Mexico and lead to armed clashes at the border.

“I don't feel really comfortable with the idea of deploying military troops and creating the possibility for an increase in violence and an escalation of the conflict,” Rep. Francis Rooney Laurence (Francis) Francis RooneyThe Hill's Convention Report: Democrats gear up for Day Two of convention The Hill's Morning Report - Presented by Facebook - Gohmert tests positive; safety fears escalate on Capitol Hill Pelosi to require masks on House floor MORE (R-Fla.), a member of the House Foreign Affairs Committee, said on CNN.

Rooney said people who cross the border illegally should be stopped and vetted through “the normal process.”

Mexico’s Ambassador to the U.S. Gerónimo Gutiérrez said on CNN his government has asked the Trump administration for a formal clarification of the president’s remarks, adding that he addressed them personally with Homeland Security Secretary Kirstjen Nielsen Kirstjen Michele NielsenDHS IG won't investigate after watchdog said Wolf, Cuccinelli appointments violated law Appeals court sides with Trump over drawdown of immigrant protections Democrats smell blood with new DHS whistleblower complaint MORE.

Nielsen, Mattis, Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE, chief of staff John Kelly John Francis KellyMORE and other top officials were set to meet at the White House on Tuesday to discuss immigration proposals.

“We are preparing for the military to secure our border between Mexico and the United States,” Trump said later Tuesday during a press conference. “We have a meeting on it in a little while with General Mattis and everybody. And I think that it's something we have to do.”

Trump has recently reverted back to the hard-line immigration stance he championed during the 2016 presidential race, backing away from his efforts to compromise with Democrats on Capitol Hill.

In a series of tweets, the president closed the door to a deal to protect immigrants brought illegally to the U.S. as children and accused Democrats of tanking it because they want to “allow open borders, drugs and crime!”

He has also called on Mexico to step up efforts to detain illegal border crossers and sounded the alarm about a “caravan” of Central American migrants heading for the U.S. border.

Trump said Tuesday he heard reports the caravan was broken up, crediting his threat to pull out of the North American Free Trade Agreement (NAFTA) if Mexico does not arrest more migrants.

“They did it because, frankly, I said you really have to do it,” the president said. “We're going to have a relationship on NAFTA; we're going to have to include security in NAFTA.”

The caravan is a group of more than 1,000 migrants traveling from Central America through Mexico toward the U.S. border. It is organized almost every year by the group Pueblo Sin Fronteras and is designed to protest U.S. immigration policy.

A spokesman for the group said not all migrants will try and cross the border and that most will stay in Mexico.

Fox News’s “Fox & Friends” discussed the caravan in a report Sunday shortly before the president tweeted about it.

The president’s tweets, however, could result in new policy proposals.

Administration officials said Monday that they are working on a legislative package designed to close what the White House sees as “loopholes” in U.S. immigration law.

It’s not clear what the package will include, though it is expected to draw on a framework from last fall that proposed making it harder to seek asylum, among other changes.

Trump has repeatedly expressed his anger that the omnibus spending bill he signed does not include his immigration demands, which could deflate his supporters ahead of November's midterm elections.

The president floated a path to citizenship for immigrants eligible for the Obama-era Deferred Action for Childhood Arrivals program, which he has ended, in exchange for funding the wall and slashing the number of people who can legally immigrate to the U.S.

“As ridiculous as it sounds, the laws of our country do not easily allow us to send those crossing our Southern Border back where they came from,” Trump tweeted Monday. “A whole big wasted procedure must take place. Mexico & Canada have tough immigration laws, whereas ours are an Obama joke. ACT CONGRESS.”

Updated at 8:35 p.m.