Nielsen framed the move as a way to toughen an immigration system that “rewards bad behavior,” including illegal drug smuggling and border crossings.

“It’s time to act,” Nielsen told reporters, adding that the deployments could begin “immediately.”

Nielsen did not share key details about the operation, including how many troops will be sent to the border, the length of the deployment or its cost.

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The move follows days of warnings from Trump about the dangers posed by illegal immigration, gangs and drugs.

Nielsen said the number of immigrants crossing the border illegally has risen since last year, when they fell following Trump’s inauguration. She said the administration is eager to act because crossings are expected to further rise in April.

“The threat is real,” Nielsen said. “This threatens not only the safety of our communities and children, but also our very rule of law, on which, as you know, our country was founded.”

The announcement sparked backlash from immigrant rights advocates, who dismissed the deployment as a political ploy to satisfy Trump’s supporters ahead of the November midterm election. “Trump’s National Guard ploy is just plain stupid,” said Frank Sharry, executive director of the liberal advocacy group America’s Voice. “This is about Trump’s incompetence, petulance and xenophobia.”

The directive will allow National Guard troops to support U.S. Customs and Border Protection agents who guard the southern frontier, but Nielsen did not specify what exactly the support role will entail.

Former Presidents George W. Bush and Obama sent guardsmen to the border during times of heightened concern about illegal immigration. Then, troops assisted border agents in constructing physical barriers, monitoring security footage and flying aircraft to monitor the border.

Nielsen said National Guard troops will not actively detain or handle migrants who cross the border illegally. An 1878 law banning federal troops from enforcing domestic laws could limit how much the guardsman can do along the border.

The Homeland Security chief said she has already begun working with border-state governors and the Defense Department on the logistics of sending National Guard units to the border.

The announcement came hours after Trump declared his administration will be taking “strong action today” to boost immigration enforcement and one day after he floated the prospect of sending troops to the border.

“Our Border Laws are very weak while those of Mexico & Canada are very strong,” Trump tweeted Wednesday morning. “Congress must change these Obama era, and other, laws NOW! The Democrats stand in our way - they want people to pour into our country unchecked....CRIME! We will be taking strong action today.”

The president has also cited a “caravan” of more than 1,000 Central Americans traveling toward the U.S. southern border as justification for a new crackdown.

The caravan, which has been the subject of reports on Fox News, is part of an annual protest organized by the group Pueblo Sin Fronteras. Most of the participants are not expected to cross the border into the U.S.

The president, who has been frustrated by the lack of progress in building his proposed border wall, met Tuesday with senior White House and Cabinet officials to discuss ideas to bolster his immigration strategy.

The ongoing planning and lack of publicly available details about the operation suggests Trump’s announcement took many in his administration by surprise.

But some aides framed it as part of an ongoing push to crack down on illegal immigration.

Senior administration officials said they are crafting a legislative package that would close “loopholes” that they say allows certain people who cross the border illegally to remain in the U.S. for too long.

The proposals would make it harder for migrants to gain asylum in the U.S. and also allow the government to speed up deportations of people apprehended near the border.

Similar language was included in an immigration framework released by the White House last fall, which was rejected by Congress.

Officials said Wednesday that Trump is weighing executive actions that could punish families who pay smugglers to bring their children illegally to the U.S.

The president is facing pressure from his base to secure a win on immigration, after Congress ignored his request for $25 billion to build the border wall he promised during the 2016 campaign.

Trump received just $1.6 billion for border barriers in the omnibus spending bill, and most of that money cannot be used to build new portions of the wall.

The president and his top aides have since tried to shift the goalposts, saying that replacements of and updates to existing wall portions also qualify as part of Trump’s wall.

“To us, it’s all new wall. If there was a wall before that needs to be replaced, it’s being replaced by a new wall,” Nielsen said on Wednesday . “This is the Trump border wall.”

Trump said Tuesday that National Guard personnel would remain along the border until his wall his built.

Updated at 10:00 p.m.