After saying he is ready to send 15,000 troops to the US-Mexico border to prevent a caravan of thousands of Central American migrants from entering the US territory, President Donald Trump on Thursday ordered the soldiers already deployed at the border to shoot the migrants if they threw rocks or stones at them.

At a White House briefing on his controversial "Operation Faithful Patriot" against what he described as "uncontrolled illegal immigration", Trump told the media that US-bound migrants heading northwards through southern Mexico had thrown rocks "viciously and violently" at Mexican police and would be met by force if they attempt the same with US troops.

"We're not going to put up with that. They want to throw rocks at our military, our military fights back," Trump warned, adding: "I told them (troops) consider it (a rock) a rifle. When they throw rocks like they did at the Mexican military and police, I say consider it a rifle."

Ahead of next Tuesday's midterm congressional elections, Trump has escalated his daily rhetoric against the migrants, brazenly accusing the opposition Democrats of being complicit in wanting to allow floods of "tough people", "rapists" and other types of threats illegally into the US through the volatile border.

Critics have condemned Trump's threats to migrants and the decision to deploy thousands of troops to the US-Mexico frontier as an expensive political stunt with an eye on next week's elections.

The US president, however, has repeatedly claimed that he is acting against "an invasion" citing the dwindling group of a few thousand impoverished migrants trying to get north, but still far from the US border, according to an AFP report.

Trump also announced that he plans to amend the US policy of allowing people to claim political asylum at the border from next week, unless they have first gone through an official border post. Those caught at the border will be held in tent camps or other facilities until they can be deported or have their requests approved, he said.

Despite criticism that such a radical rethink to asylum policies could violate current US laws, Trump is in no mood to be convinced otherwise.

"This is totally legal. No, we're stopping people at the border. This is an invasion, and nobody is even questioning that," he said. "We'll be doing an executive order sometime next week," he said, without providing much detail.

US Defense Secretary Jim Mattis has backed Trump's position while rejecting the conjectures about the move being a political stunt.

"The support that we provide to the Secretary for Homeland Security is practical support based on the request from the commissioner of Customs and Border Police," Mattis told the media, adding: "We don't do stunts in this department."

According to reports Pentagon has so far authorized more than 5,200 troops to go to the border after Trump's latest decision. Another 2,000 troops are on standby, and some 2,000 National Guardsmen are already in the border region to support agents there, taking the total number of troops to more than 9,000 already and just 6,000 short of the final number approved by Trump for "Operation Faithful Patriot".

Critics have also pointed out that deployment of 15,000 troops would be equivalent to the size of the US military's presence in Afghanistan, and nearly three times their numbers in Iraq. It is also more than three times as many troops as are engaged in fighting the Islamic State extremist group in Syria.

Regardless of the political hullabaloo over the issue, the US government figures released on Wednesday showed that only about 400,000 people have been apprehended at the border in 2018, down from around 1.6 million in 2000.

According to the US Border Patrol data, the number of illegal migrants crossing the border from Mexico into the US rose steadily since the 1960s; but after peaking at more than 1.6 million in 2000, the numbers fell sharply and are now on the level of the early 1970s. Immigration has been a crucial topic ever since Trump took charge in 2017. 

Earlier this year there was a global outrage as well as from across the US political spectrum over the reported separation of children from migrant parents after their detention at the US border. A Quinnipiac University poll showed over 66 percent of Americans opposed separating children from asylum seekers.

Even Trump's plan to build a wall along the border with Mexico has elicited little support among Americans.

According to a survey by Pew Research last year, 62 percent of Americans opposed the idea with only 35 percent voting in favor of the wall. 

(With input from agencies)