US President Donald Trump said he would use military forces to protect the border with Mexico until the wall is completed. Picture: Xinhua/Ting Shen

Washington - President Donald Trump said on Tuesday he would use U.S. military forces to protect the border with Mexico until a long-promised wall is completed and "proper security" is in place.

Trump, who earlier threatened to halt US foreign aid to Honduras and other countries unless they stopped a "caravan" of Central American migrants headed to the United States, called use of the military at the southern border "a big step."

Trump has railed against more than 1 200 Central American migrants on a 2 000-mile (3 200-km) journey from the Mexico-Guatemalan border, and reiterated threats to derail the North American Free Trade Agreement (NAFTA) if they are not stopped.

"We will be doing things with Mexico, and they have to do it, otherwise I’m not going to do with the NAFTA deal," Trump told reporters at the White House. He said if the "caravan" reaches the US border "our laws are so weak and so pathetic ... it's like we have no border."

Mexican officials stepped up efforts on Tuesday to process the dwindling group and determine whether they had the right to stay in Mexico or be returned to their countries of origin. Mexico has said such "caravans" of mostly Central Americans, including many escaping violence in Honduras, have occurred since 2010.

"Until we can have a wall and proper security we’re going to be guarding our border with the military," Trump said. He said he would be meeting soon with US Defense Secretary Mattis and others to discuss the idea.

In a post on Twitter earlier on Tuesday, Trump said the caravan "heading to our 'Weak Laws' Border, had better be stopped before it gets there. Cash cow NAFTA is in play, as is foreign aid to Honduras and the countries that allow this to happen. Congress MUST ACT NOW!"

Some members of Congress said they were uncomfortable with the idea of using the military at the border. Democratic Senator Brian Schatz said Trump should have to ask for approval from Congress.

"I think we should put that new law to a vote in the Senate as soon as possible," he said on Twitter. “I predict fewer than 20 votes."

Republican Representative Francis Rooney, a member of the Foreign Affairs Committee in the US House of Representatives, said there was a risk of increased violence. "These people should be stopped at the border and vetted out, just the normal process, and we should have plenty of agents down there to do that," Rooney told CNN.

US presidents have ordered National Guard forces to the border in the past. Under President George W. Bush, National Guard forces from all 54 US states and territories were used between 2006 and 2008 for things like border-related intelligence analysis but did not have a direct law enforcement role, according to the Pentagon.

On Monday, the Republican president railed against Democrats over immigration and again pressed US lawmakers to pass legislation to build his long-promised border wall between the United States and Mexico.

Despite months of efforts, no immigration deal has emerged in the Republican-led Congress, where lawmakers are not expected to pass much major legislation ahead of November's midterm congressional elections.