Frustrated by slow action on a major campaign promise, US president Donald Trump has said he wants to use the military to secure the US-Mexico border until his promised border wall is built.

Mr Trump told reporters he has been discussing the idea with Defence 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,” Mr Trump said, calling the move a “big step.”

It was not immediately clear exactly how the proposal would work or what kind of troops Mr Trump wanted to deploy. But the White House later said Trump wanted to mobilise the National Guard.

Federal law prohibits the use of active-duty service members for law enforcement inside the US, unless specifically authorised 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. The White House counsel’s office has been working on the idea for several weeks, according to a senior official.

Mr Trump has been annoyed by the lack of progress on building what was the signature promise of his campaign: a “big, beautiful wall” along the Mexican border. He has previously suggested using the Pentagon’s budget to pay for building the wall, arguing it is a national security priority, despite strict rules that prohibit spending that is not authorised by Congress.

At the Pentagon, officials struggled throughout the day to answer questions about the plan, including rudimentary details on whether it would involve National Guard members.

But the administration appeared to be considering a model similar to a 2006 operation in which President George W Bush deployed National Guard troops to the southern border.

Under Operation Jump Start, 6,000 National Guard troops were sent to assist the border patrol with non-law enforcement duties while additional border agents were hired and trained. Over two years, about 29,000 National Guard forces participated, as forces rotated in and out. The Guard members were used for surveillance, communications, administrative support, intelligence, analysis and the installation of border security infrastructure.

In addition, President Barack Obama sent about 1,200 National Guard troops to the US-Mexico border in 2010 to beef up efforts to battle drug smuggling and illegal immigration.

Texas has also deployed military forces to its 800-mile border with Mexico. Former Texas Governor Rick Perry, now serving as Mr Trump’s energy secretary, sent 1,000 Texas National Guardsmen to the Rio Grande Valley in 2014 in response to a sharp increase in Central American children crossing the border alone.

Mr Trump met on Tuesday with top administration officials, including Mr Mattis, Homeland Security Secretary Kirstjen Nielsen and Attorney General Jeff Sessions, to discuss the administration’s strategy to address what White House spokeswoman Sarah Huckabee Sanders described as “the growing influx of illegal immigration, drugs and violent gang members from Central America.”

In addition to mobilising the National Guard, Mr Trump and senior officials “agreed on the need to pressure Congress to urgently pass legislation to close legal loopholes exploited by criminal trafficking, narco-terrorist and smuggling organisations,” Ms Sanders said.

The meeting and comments came amid a flurry of tweets by the president on the subject over the last several days.

Mr Trump has been fixated on the issue since he grudgingly signed a spending bill last month that includes far less money for the wall than he had hoped for.

On Tuesday, Mr Trump appeared to take credit for halting a caravan of about 1,100 migrants, many from Honduras, who had been marching along roadsides and train tracks in the southern Mexican state of Oaxaca.

“I said (to Mexican officials), ‘I hope you’re going to tell that caravan not to get up to the border.’ And I think they’re doing that because, as of 12 minutes ago, it was all being broken up,” he said.

But the caravan of largely Central American migrants had never intended to reach the US border, according to organiser Irineo Mujica. It was meant to end at a migrants’ rights symposium in central Mexico later this week.

The caravan stopped to camp at a sports field in Oaxaca over the weekend. Mexican immigration officers have been signing them up for temporary transit visas, which would allow them to travel to the US border, possibly to seek asylum, or to seek asylum status in Mexico.