As around 3000 Hondurans hoping to make it to the United States made their way through Guatemala, attention turned to Mexico yesterday.

US President Donald Trump threatened to close the US-Mexico border if authorities there fail to stop them - a nearly unthinkable move that would disrupt hundreds of thousands of legal freight, vehicle and pedestrian crossings each day.

With less than three weeks before the November 7 US mid-term elections, Trump seized on the migrant caravan to make border security a political issue and energise his Republican base.

"I must, in the strongest of terms, ask Mexico to stop this onslaught - and if unable to do so I will call up the US Military and CLOSE OUR SOUTHERN BORDER!" Trump tweeted, adding that he blamed Democrats for what he called "weak laws!"

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The threat followed another one earlier in the week to cut off aid to Central American countries if the migrants weren't stopped. Trump made a similar vow over another large migrant caravan in April, but didn't follow through after it largely petered out in Mexico.

Yesterday, Mexico dispatched additional police to its southern border after the Casa del Migrante shelter on the Guatemalan side of the border reported that hundreds of Hondurans had already arrived there.

Apparently pleased with that response, Trump retweeted a BuzzFeed journalist's tweet of a video clip showing the police deployment, adding his own comment: "Thank you Mexico, we look forward to working with you!"

Hundreds of Honduran migrants stand at the shore of the Suchiate river on the border between Guatemala and Mexico, in Tecun Uman, Guatemala. Photo / AP
Hundreds of Honduran migrants stand at the shore of the Suchiate river on the border between Guatemala and Mexico, in Tecun Uman, Guatemala. Photo / AP

Mexican officials said the Hondurans would not be allowed to enter as a group and would either have to show a passport and visa - something few have - or apply individually for refugee status, a process that can mean waiting for up to 90 days for approval. Migrants caught without papers would be deported, they said.

Marcelo Ebrard, who is set to become Mexico's Foreign Relations Secretary when President-elect Andres Manuel Lopez Obrador takes office on December 1, said Trump's tweets need to be understood in the context of the upcoming US mid-term elections.

"The electoral process is very near, so he is making a political calculation," Ebrard said.

Current Foreign Relations Secretary Luis Videgaray said of Trump's comments: "Nobody likes them. There's no reason to give them greater transcendence or importance.

"What is important to us is the migrants, respect for human rights, their due protection, particularly the most vulnerable."

Still, the idea that Mexico could close its porous southern border - or that the US would choke off the lucrative trade and other traffic between the two nations - strained the imagination.

"There would be huge economic impacts for both the United States and Mexico ... but limited effect on illegal immigration," said Andrew Selee, president of the Migration Policy Institute.

Stephanie Leutert, director of the Mexico Security Initiative at the University of Texas at Austin, said she interpreted the tweet to mean Trump could send troops not to ports of entry but elsewhere where the illegal crossings take place. "If that's the case, I don't think Mexico should be too worried because in a sense ... it's the same kind of thing US administrations have been doing for a long time," Leutert said.

Like Guatemala and Honduras, Mexico is a country of many migrants, raising the question of whether the political will exists for a confrontation.

Lopez Obrador wants to avoid repression against migrants and also to avoid angering the US. He said this week that Mexico would offer jobs to Central Americans. "Anyone who wants to work in our country ... will have a work visa," he said.

By yesterday, the caravan had dispersed a bit, with the youngest and strongest of the migrants walking ahead together, some boarding buses or trying to hitch rides.

Honduran migrants bound to the U.S border climb into the bed of a truck in Zacapa, Guatemala. Photo / AP
Honduran migrants bound to the U.S border climb into the bed of a truck in Zacapa, Guatemala. Photo / AP

They carried belongings in everything from backpacks to rolling suitcases to black plastic garbage bags. At the side of a highway, a woman in blue changed a baby's diaper on a blanket spread on the ground.

Juan Escobar, 24, said he had heard about Trump's comments but said they would not dissuade the migrants from continuing their journey.

"Only God on high can stop us," Escobar said.

Carlos Lopez, 27, said he was concerned by Trump's threats, but "you have to keep fighting".

Trump also warned that he prioritises border security over even the recently struck trade deal to replace Nafta, the United States-Mexico-Canada Agreement, or USMCA.

"The assault on our country at our Southern Border, including the Criminal elements and DRUGS pouring in, is far more important to me, as President, than Trade or the USMCA. Hopefully Mexico will stop this onslaught at their Northern Border," Trump tweeted.

Analysts didn't see the pact as being in imminent danger, though trade lawyer Daniel Ujczo of Dickinson Wright said there is "a significant concern" that Trump could hold the agreement hostage over future issues.

"Leaders around the world are sceptical that any deal with this US Administration is actually final," Ujczo said, "particularly one such as the USMCA where the ink has not been put to the signature line."

US-bound migrant caravans have been going on for years - with travelling in numbers seen as offering protection from assaults, robberies, even shakedowns by police. They're also a cheaper alternative to the US$7000 ($10,655) to US$10,000 that smugglers charge for passage to the border, Leutert noted. Still, it wasn't until this year that the caravans received widespread attention.

"There have been these caravans through the years, but they become prominent because the President tweets about them," Selee said.

He predicted that, like the caravan in April, Mexico will respond with measures such as granting asylum to some migrants who qualify while deporting others who don't, perhaps not eliminating the caravan entirely but significantly reducing its size before it reaches the US border.

But the direct, public pressure from Trump puts Mexico, already an uneasy ally for the last two years, in an uncomfortable spotlight.

"Ironically, the way President Trump responds to these caravans makes it harder for the Mexican Government to co-operate with the US on immigration enforcement," Selee said. "There is a lot of disposition in both the current and the incoming Mexican Government to co-operate with the US on some aspects of immigration control. But it becomes much harder when President Trump makes this a political issue in which he bashes Mexico."

- AP