WASHINGTON — President Trump and border-state governors are working to immediately deploy the National Guard to the US-Mexico border to fight illegal immigration, Homeland Security Secretary Kirstjen Nielsen said Wednesday.

‘‘The threat is real,’’ Nielsen said, adding that Trump was signing a proclamation to put the deployment into effect. ‘‘It’s time to act.’’

The announcement came hours after Trump pledged ‘‘strong action today’’ on immigration and a day after he said he wanted to use the military to secure the southern border until his long-promised border wall is erected.

Nielsen said she had been in touch with governors of the Southwest border states and was working with them to develop agreements that will oversee where and how many National Guard members will be deployed. She suggested that some troops could begin arriving as soon as Wednesday night, though other administration officials cautioned that details on troop levels, locations, and timing were still being worked out.

‘‘We do hope that the deployment begins immediately,’’ she said.

Trump has been frustrated by slow action on building his ‘‘big, beautiful wall’’ along the Mexican border — the signature promise of his campaign — as well as a recent uptick in illegal border crossings, which had plunged during the early months of his presidency. He has also been fixated on the issue of border security since he grudgingly signed a spending bill last month that includes far less money for a wall than he had hoped for.

Federal law prohibits the use of active-duty service members for law enforcement inside the United States, unless specifically authorized by Congress. But over the past 12 years, presidents have twice sent National Guard troops to the border to bolster security and assist with surveillance and other support.

Nielsen said the effort would be similar to a 2006 operation in which President George W. Bush deployed troops to help Customs and Border Protection personnel with non-law enforcement duties while additional border agents were hired and trained. President Barack Obama sent about 1,200 troops in 2010 to beef up efforts against drug smuggling and illegal immigration.

Nielsen said her department had developed a list of locations where it would like assistance and was discussing with the governors how to facilitate the plans. She declined to say how many personnel would be needed or how much the operation would cost. ‘‘It will be as many as is needed to fill the gaps that we have today,’’ she said.

One congressional aide said lawmakers anticipate 300 to 1,200 troops will be deployed, at a cost of at least $60 million to $120 million a year.

The Republican governors of Texas and Arizona applauded the move. New Mexico Governor Susana Martinez, also a Republican, did not immediately comment. Democratic Governor Jerry Brown of California, who has been sparring with Trump over immigration policies, is likely to oppose it.

Trump revealed Tuesday that he had been discussing the idea of using the military at the border with Defense Secretary Jim Mattis.

‘‘We’re going to be doing things militarily. Until we can have a wall and proper security, we’re going to be guarding our border with the military,’’ Trump said, calling the move a ‘‘big step.’’

He spent the first months of his presidency bragging about a dramatic drop in illegal border crossings, which some DHS officials had dubbed the ‘‘Trump effect.’’ Indeed, arrests at the border last April were at the lowest level since the Department of Homeland Security was created in 2003, and the 2017 fiscal year saw a 45-year low for Border Patrol arrests.

But the numbers have been slowly ticking up since last April and are now on par with the totals for many months of the Obama administration. Statistics show 36,695 arrests of people trying to cross the Southwest border in February 2018, up from 23,555 in the same month of the previous year.

Trump’s new focus on hard-line immigration policies appears aimed, at least in part, to drawing a political contrast with Democrats heading into the midterm elections. He has also been under growing pressure from conservative backers who have accused him of betraying his base for not delivering on the wall.