FORT MYERS, United States - US President Donald Trump further hardened his pre-election anti-immigration rhetoric in a Florida campaign stop on Wednesday, after threatening to deploy as many as 15,000 soldiers on the Mexican border - equal to the size of the US contingent in Afghanistan.

"They got a lot of rough people in these caravans. They are not angels," he said in Fort Myers, referring to migrants from poor Central American countries moving toward the United States in hopes of a better life or to escape violence.

"We're gettin' prepared for the caravan, folks," he said.

Brushing aside accusations that his divisive rhetoric on immigration is stoking extremism, Trump made the troop announcement before flying to Florida for the last stage of campaigning ahead of next Tuesday's midterm elections.

He will host 11 rallies across eight states in the next six days.

On Tuesday, Trump announced that more than 5,000 active duty soldiers were being sent.

That was already highly unusual, but on Wednesday, he told reporters at the White House: "We'll do up to anywhere between 10 and 15,000 military personnel."

Trump frequently describes illegal immigrants－a tiny minority of whom have formed groups to attempt walking hundreds of miles to the US border－as "rapists" and "thugs".

"It's a dangerous group of people," Trump said of the latest group of a few thousand migrants, who are still deep inside Mexico far from their goal. "They're not coming into our country."

The situation, according to the Department of Homeland Security, is "an unprecedented crisis".

However, the department's own figures show that the number of illegal immigrants intercepted in 2018 was only 400,000, a mere 25 percent of the 1.6 million figure in 2000.

Meanwhile, thousands of weary Central Americans in a migrant caravan aiming to reach the US had their visions of quick transport hundreds of miles ahead to Mexico City dashed on Wednesday as dozens of hoped-for buses failed to materialize.

The migrants took the day off from walking and hitching rides in packed trucks from small town to small town as representatives tried to negotiate rides for all 4,000 or so in hope of relief from the long and exhausting grind.

But as the day wore on there was no sign Mexican authorities intended to accede to the demand, and by evening leaders acknowledged it wasn't going to happen.

"The attempt to travel by bus failed," coordinator Walter Cuello said.

White House press secretary Sarah Huckabee Sanders on Wednesday praised Mexico for stopping the migrants from getting rides. "Mexico has stepped up in an unprecedented way," Sanders told Fox News. "They have helped stop a lot of the transportation means of these individuals in these caravans, forcing them walking. They have helped us in new ways to slow this down, to break this up and keep it from moving as aggressively toward the United States."

AFP/AP