WASHINGTON — The Trump administration announced a hastily assembled plan Wednesday to deploy National Guard troops along the southwestern border, hoping to make good on a promise the president made a day earlier that caught many in the military by surprise.

Homeland Security Secretary Kirstjen Nielsen said that troops would be deployed “immediately” and that she had already spoken to four governors whose states share the border. But by day’s end, none of the four had publicly committed to deploy a specific number of guard units.

At least one, California Gov. Jerry Brown, heard from Nielsen about the issue for the first time on Wednesday, according to a spokesman. Without key details — including the number of troops, the duration of their deployment and the cost — the Democratic governor was unwilling to commit to the effort, his spokesman said.

The administration’s request “will be promptly reviewed to determine how best we can assist our federal partners,” California National Guard spokesman Lt. Col. Tom Keegan said in a statement.

Administration officials who described their plans to reporters were insistently vague about the number of Guard personnel who might be sent to the border. And while they offered a detailed account of what they see as the main current problem there — a rising number of Central American refugees making claims for asylum — they left unclear what troops could do about it.

“The president is frustrated” over continued illegal immigration, Nielsen said, adding, “He has been very clear that he wants to secure our border.”

Trump signed a proclamation Wednesday night to allow the deployment, but the decisions have to be made in conjunction with the governors of the four border states, Nielsen said.

The hurried moves came a day after Trump surprised the military and some of his own staff with talk of deploying troops to guard the border.

“Until we can have a wall and proper security, we’re going to be guarding our border with the military. That’s a big step,” Trump said at a White House ceremony Tuesday.

Later that day, White House officials said that what Trump was actually referring to was deployment of National Guard units, much as President George W. Bush did in 2006 and President Barack Obama did in 2010.

National Guard personnel would not be allowed to arrest people crossing the border — something barred by federal law — but could provide support to the Border Patrol. In previous deployments, Guard troops have been used to help build roads, keep watch for illegal crossings and scout smuggling routes.

Trump’s call to bring in the military mirrored other recent policy and personnel announcements that have forced advisers to scramble. It came after several days in which the president stewed over the lack of congressional funding to build his promised border wall and suffered criticism from some of his nationalist allies for failure to win more money for the project.

In briefing reporters, Nielsen said the administration had recently seen an uptick in illegal border crossings after a steep decline last year. The numbers are not definitive, and monthly figures often fluctuate. But officials appear concerned that illegal crossings could mount quickly in the spring, the season when they typically peak.

Smuggling gangs “paused” after Trump took office, but “these illicit smuggling groups saw that our ability to actually remove those who come here illegally did not keep pace. They saw that there were loopholes they could exploit to avoid detention and removal,” she said.

But Nielsen left unclear what the National Guard would be able to do to help that situation, which she blamed primarily on people who claim a “credible fear” of persecution in their home countries and ask for asylum in the U.S. In many such cases, asylum seekers cross the border, quickly surrender to the Border Patrol and ask for refuge in the U.S.

Under U.S. law, people who have an asylum claim must be given a hearing, and in many cases are released until their court date. Families with minor children cannot be held in detention for more than 20 days under certain circumstances. Government figures show that a large majority of those who are released do show up for their hearings.

The government could keep a larger number in detention, but that would be extremely expensive. The daily detention cost is about $164 per person. Immigration officials are authorized to hold up to 34,000 people at one time, bringing the federal government’s annual budget for immigration detention to about $2 billion.

Before 2013 about 1 percent of border crossers claimed asylum; now about 10 percent do, Nielsen said, suggesting that much of the increase involved false claims.

People “know that if they use the magic words, ‘credible fear’ ... they will be released, with work permits, into the interior for many years before they see a judge,” she said.

The administration would once again ask Congress to change immigration law to allow faster processing of claims and speedier deportations, she added.

Another administration official who briefed reporters on the condition of anonymity promised political repercussions for Democrats who are likely to oppose the White House’s legislative proposals, suggesting Trump would hope to use the issue to motivate conservative voters in the November congressional elections.

Nielsen said the administration would work with governors to determine how many Guard troops to deploy. The number “will be as many as is needed to fill the gaps today,” she said. Other administration officials said they would leave that decision to the military.

States already have some Guard members positioned along the border. The California National Guard has about 250 people working to stop illegal drugs from getting into the state, including 55 helping along the border.

Arizona Gov. Doug Ducey and Texas Gov. Greg Abbott, both Republicans, welcomed Trump’s action but stopped short of specifying how many personnel their states would activate.

“Arizona welcomes the deployment of National Guard to the border,” Ducey wrote on Twitter. “Washington has ignored this issue for too long and help is needed. For Arizona, it’s all about public safety.”

Abbott pointed out in a statement that since he became governor in 2015, the Texas National Guard had maintained a continuous presence on the border.

“Texas will continue to implement robust border security efforts, and this partnership will help ensure we are doing everything we can to stem the flow of illegal immigration,” he said. He did not mention deploying any additional resources, although state officials said that remained a possibility.

A spokesman for Republican New Mexico Gov. Susana Martinez told the Associated Press on Wednesday that she appreciated the Trump administration’s efforts to involve states as the federal government takes steps to better secure the border.

Immigration advocates accused Trump of manufacturing a crisis to demonize immigrants.

“This is another impulsive reaction to not getting his wall fully funded and a maneuver to distract the American public from the crisis he created for 800,000 young immigrants when he ended DACA, whose lives and futures are at risk,” said Astrid Dominguez, director of the ACLU Border Rights Center, referring to the Obama-era Deferred Action for Childhood Arrivals program that protected young immigrants from deportation.

“Military troops don’t belong at the border; there is no security crisis, and this deployment wastes valuable time and resources,” she said.

Mexican Foreign Minister Luis Videgaray said Wednesday that his government had asked the White House for an explanation of its intentions and would formulate a response “in defense of our sovereignty and national interest.”