President Donald Trump threatened on Thursday to deploy the military and close the southern US border if Mexico does not halt a caravan of Central America migrants heading north, raising the risk of huge disruptions to trade.

Stretching almost 2,000 miles (3,200 km), the US-Mexican border is one of the busiest in the world, processing thousands of commuters daily and much of the half a trillion dollars of annual trade between Mexico and the United States.

“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 wrote on Twitter.

Several thousand Honduran migrants moved this week through Guatemala, and some were trying to cross to Mexico on Thursday, local media said. Some hope to eventually enter the United States to escape violence and poverty.

Mexican President-elect Andres Manuel Lopez Obrador said in the northern city of Saltillo on Thursday that his designated foreign minister, Marcelo Ebrard, would speak with US Secretary of State Mike Pompeo on Friday to explore a solution to the caravan issue.

“An agreement can be reached,” said Lopez Obrador, who takes office in December. “We’re going to take care of our relationship with the United States government. It’s very important to have a relationship of friendship.”

Trump told a political rally in Montana on Thursday night that he wanted “to thank the Mexican government because they are stopping it hopefully before it ever gets to Mexico.”

Central American migrants hiked from Honduras through muddy jungle and residential streets, some toting babies along with backpacks.

In Guatemala City, where migrant shelters filled with people, waves of people departed at daybreak on roads leading to Mexico. The nearest border is about 110 miles (177 km) away.

“If we don’t get across, we’re going to try the same thing again,” said Gustavo Perez, a builder from Honduras, speaking at a shelter in Guatemala City.

“We hope that in this big caravan group, they let us in,” he added, referring to the United States.

Mexico’s peso weakened over 1.5 per cent against the dollar on Thursday, partly driven by Trump’s comments, analysts said.

Mexico’s Foreign Ministry said in a statement it would ask the United Nations refugee agency for help coordinating with Central American governments on people seeking refugee status at Mexico’s southern border.

Election issue 

Trump, who has sought to curtail immigration and build a border wall on the Mexican border, threatened this week to halt aid if Central American governments did not act.

Trump ran for office vowing to toughen US immigration policies and build a wall along the US border with Mexico.

Illegal immigration is likely to be a top issue on November 6 US congressional elections.

Trump said migrants in the caravan were being used by his political opponents.

“A lot of money has been passing to people to come up to try and get to the border by Election Day,” he said, without providing evidence.

Frustrated by Congress’ failure to fully fund his proposed wall at the border with Mexico, Trump in April ordered National Guard personnel to help secure the border in four southwestern US states.

In a string of tweets, Trump also said the border issue was more important to him than the new trade deal with Mexico to replace the North American Free Trade Agreement.

“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 wrote. He was referring to the United States-Mexico-Canada Agreement, which is awaiting ratification.

US Representative Michael McCaul, the Republican chairman of the House Homeland Security Committee and a member of the House Foreign Affairs Committee, said US border security was a key election issue.

“We have to secure that border once and for all,” McCaul told Fox News in an interview, adding that he hoped Congress could tackle the issue of Trump’s planned wall after the November 6 election.

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

The caravan has been growing steadily since it left the violent Honduran city of San Pedro Sula on Saturday. There are no official estimates of the size of the group.

Guatemalan President Jimmy Morales said on Wednesday his government dismissed threatened constraints placed on foreign aid.

He said he had spoken with Honduran President Juan Orlando Hernandez about ensuring the migrants who want to return home can do so safely.

Ebrard, Mexico’s incoming foreign minister told local radio that he was not surprised by Trump’s comments.

“It was predictable, and it’s also very close to the election. He’s making a political calculation,” Ebrard said, adding that migrants arriving in Mexico without a visa would need to apply for refugee status.

Pompeo is due to meet with outgoing Mexican President Enrique Pena Nieto in Mexico City on Friday.