Donald Trump has pledged "strong action today" on immigration, a day after he said he wants to use the military to secure the US-Mexico border until his "big, beautiful wall" is erected.
In an early-morning tweet, the president said "Our Border Laws are very weak" and claimed Democrats "stand in our way" of new laws. He added: "We will be taking strong action today."
He did not offer further details and the White House did not immediately respond to requests for comment.
Mr Trump told reporters on Tuesday that he has been discussing the idea of using the military at the border 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 he wanted to deploy, but the White House later said he 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 source.
Mr Trump has been frustrated by slow action on building a wall along the Mexican border.
He has previously suggested using the Pentagon's budget to pay for 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 on Tuesday 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 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, Barack Obama sent about 1,200 National Guard troops to the Mexico border in 2010 to beef up efforts against drug smuggling and illegal immigration.
Texas also deployed military forces to its 800-mile border with Mexico.
Former Texas governor Rick Perry, now 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.
On Tuesday Mr Trump met senior 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".
As well as 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.