President Donald Trump says the US will begin curtailing tens of millions of dollars in aid to three Central American nations, calling a caravan of migrants bound for the US a "national emergency".

Key points: Mr Trump and fellow Republicans seek to elevate immigration issues ahead of elections

Mr Trump and fellow Republicans seek to elevate immigration issues ahead of elections US has decreased aid to Guatemala, Honduras and El Salvador by almost 40 per cent since Mr Trump took office

US has decreased aid to Guatemala, Honduras and El Salvador by almost 40 per cent since Mr Trump took office Caravan's numbers continue to grow, UN estimates it is currently at around 7,200 people

"Guatemala, Honduras and El Salvador were not able to do the job of stopping people from leaving their country and coming illegally to the US," Mr Trump tweeted.

"We will now begin cutting off, or substantially reducing, the massive foreign aid routinely given to them."

Mr Trump and his fellow Republicans have sought to elevate the caravan, which has made its way into Mexico bound for the US border, and immigration as campaign issues ahead of the mid-term elections, in which his party is fighting to maintain control of the US Senate and House of Representatives.

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"Remember the Midterms!" Mr Trump wrote in Twitter posts decrying the caravan and attacking Democrats on immigration.

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Mr Trump, who has taken a hard line towards illegal immigration since taking office last year, also wrote that he had alerted the US military and border patrol "that this is a National Emergency" though he did not say what actions he was planning.

Sorry, this video has expired Thousands of migrants inch closer to America

Mr Trump travelled to Texas, a key border state, on Monday (local time) to campaign for Republican senator Ted Cruz, who is seeking re-election.

On Monday night (local time), he spoke to a big crowd wearing Make America Great Again hats and waving signs, some with Mr Trump's new catchphrase, "Jobs vs mobs".

Mr Trump said Democrats were "encouraging millions of illegal aliens to break our laws, violate our borders and overwhelm our nation".

He called the caravan an "assault on our country" and suggested, without citing evidence, "Democrats had something to do with it".

He added: "We need a wall built fast."

A migrant family walks past a girl as she sleeps along a sidewalk in Tapachula city centre. ( Reuters: Adrees Latif )

US has already made big cuts to Central American aid

Since Mr Trump became President last year, the United States has moved to sharply decrease aid to Central America.

In 2016, the United States provided some $US131.2 million ($185.18 million) in aid to Guatemala, $98.3 million to Honduras, and $67.9 million to El Salvador, according to official US data.

By next year, those sums were projected to fall to $69.4 million for Guatemala, $65.8 million for Honduras, and $45.7 million in the case of El Salvador.

Combined, the cuts amount to a reduction of almost 40 per cent for the three nations.

Mr Trump was not specific about how much more he wanted to cut.

Migrants hold their hands out for food donations in Tapachula city centre. ( Reuters: Adrees Latif )

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Lieutenant Colonel Jamie Davis, a Pentagon spokesman, said that while National Guard troops are currently supporting the Department of Homeland Security on the border, the Pentagon had not been asked to provide additional support.

There are currently 2,100 National Guard troops along the border in Texas, New Mexico, Arizona and California, according to the Pentagon.

Representatives for the White House and the US Border Patrol did not immediately reply to requests for comment.

Migrants say Mr Trump using caravan for political ends

Thousands of migrants, mostly Honduran, massed late on Sunday in the Mexican city of Tapachula near the Guatemalan border, local police said.

They stretched out overnight on rain-soaked sidewalks, benches and public plazas in the city, worn down by another day's march under a blazing sun.

Some huddled under a metal roof in the city's main plaza.

Others lay exhausted in the open air, with only thin sheets of plastic to protect them from ground soggy from an intense evening shower, while some had no plastic at all.

Honduran migrants sleep in a public plaza in Tapachula featuring a statue of Mexican national hero Miguel Hidalgo. ( AP: Moises Castillo )

About another 1,000 migrants were walking toward them from Ciudad Hidalgo further south, according to a Reuters witness.

Members bristled at suggestions there could be terrorists among them and said the caravan was being used for political ends by Mr Trump.

Seeking to escape violence and poverty in their home countries, the migrants — men, women and children — have defied threats by Mr Trump that he will close the US-Mexican border if they advanced, as well as warnings from the Mexican government.

Police in riot gear shadowed the caravan's arrival along a southern highway but did not impede the migrants' journey.

The caravan's numbers have continued to grow, and the United Nations estimated that it comprised some 7,200 people, "many of whom intend to continue the march north".

However, they were still some 1,800 kilometres from the nearest border crossing — McAllen, Texas — and the length of their journey could double if they go to Tijuana-San Diego, as many did in another caravan earlier this year.

Central American migrants wait on the bridge in Ciudad Hidalgo that connects Guatemala and Mexico. ( Reuters: Edgard Garrido )

'Hunger and death' driving people to join caravan

"It is a shame that a President so powerful uses this caravan for political ends," said Irineo Mujica of the group Pueblo Sin Fronteras — People Without Borders — which works to provide humanitarian aid to migrants.

Some have questioned the timing so close to the vote and whether some political force was behind it, though by all appearances it began as a group of 160 who decided to band together in Honduras for protection and snowballed as the group moved north.

"No one is capable of organising this many people," Mr Mujica said, adding that there are only two forces driving this: "hunger and death".

Residents hand out water bottles to Central American migrants at the main plaza in Tapachula, Mexico. ( AP: Moises Castillo )

A team of AP journalists traveling with the caravan for more than a week has spoken with Hondurans, Guatemalans and Salvadorans, but has not met any Middle Easterners, who Mr Trump suggested were "mixed in" with the Central American migrants.

It was clear though that more migrants were continuing to join the caravan.

Denis Omar Contreras, a Honduran-born caravan leader with People Without Borders said Mr Trump should stop accusing the caravan of harbouring terrorists.

"There isn't a single terrorist here," he said.

"We are all people from Honduras, El Salvador, Guatemala and Nicaragua. And as far as I know there are no terrorists in these four countries, at least beyond the corrupt governments."

Sorry, this video has expired Migrants trek under Mexico heat en route to US

Reuters/AP