A caravan of thousands of Central American migrants heading through Mexico for the United States will find it "difficult" to reach its destination, Mexican President Enrique Pena Nieto said Monday.

Only about 1,000 of the caravan's estimated 7,500 participants have heeded Mexico's call to seek refugee status in the country. Their refusal to thus legalize their situation will make it difficult for them to reach the US or to stay in Mexico, the president said at a business summit.

Many of the migrants shun the procedure, because it would take months and because most of them do not want to stay in Mexico.

The possibilities of the caravan reaching the US are "extremely remote," Interior Minister Alfonso Navarrete told journalists.

Pena Nieto and Navarrete made their comments after US President Donald Trump warned of a looming national emergency over the caravan.

The president said that the US would take measures against Guatemala, Honduras and El Salvador for failing to stop the caravan early on, and "begin cutting off, or substantially reducing, the massive foreign aid routinely given to them."

The caravan, which took off from northern Honduras on October 13, meanwhile continued its journey northwards after passing the night in Tapachula, a city in south-western Mexico.

"Sadly, it looks like Mexico's Police and Military are unable to stop the Caravan heading to the Southern Border of the United States," Trump said in one tweet.

"Criminals and unknown Middle Easterners are mixed in. I have alerted Border Patrol and Military that this is a National Emergy [sic]. Must change laws!" Trump added.

Migrants who spent the night in parks and squares in Tapachula denied Trump's accusations.

"We are not criminals, Donald Trump. We are not terrorists," said Denys Omar Contreras from Honduras, a spokesman for the caravan. "The only weapons we carry are a desire to get ahead and to protect our lives," he added.

"We are migrants, we are not criminals! We are international workers!" some of the migrants chanted.

Many of the migrants are moving on foot, spraying themselves with water and using umbrellas to shade themselves from the scorching sun. Others travel in trucks or other vehicles.

Some of the migrants displayed Honduran flags, though the caravan also includes citizens of Guatemala and El Salvador.

"We are fleeing gang violence. We are not only fleeing because we have no work, we live in hell," Contreras said.

"Here we go, we'll see what happens. I hope to reach my goal," said a woman named Marisol, who was travelling with eight members of her family.
"It has been tough," said Noe Reyes from Honduras. But, he added, the goal was clear: "To move on, always."

Guatemalan police meanwhile gathered in the eastern town of Quezaltepeque to block an another caravan of about 1,500 Honduran migrants, according to local media.

UN Secretary General Antonio Guterres is urging countries to work with the International Organization for Migration (IOM) and the UN's refugee agency UNHCR to respect the rights and dignity of the migrants in the caravan, spokesman Farhan Haq said.

The IOM and UNHCR both have people on the ground working with partners to offer humanitarian assistance and legal advice, according to Haq.

Trump has threatened to call in the US military, though what legal role the armed forces could play remains unclear.

Since taking office last year, Trump has stepped up measures to curtail the number of refugees and migrants entering the US. 
Analysts say tensions over migration could increase after leftist Andres Manuel Lopez Obrador succeeds Pena Nieto on December 1.

Migrants do not leave their homes "for fun, but out of necessity," Lopez Obrador's website quoted him as saying on Sunday.

The president-elect said he would propose that the United States, Canada and Mexico invest in the development of Central American countries to discourage their citizens from seeking better lives in the US.

That amounts to an opposite approach from Trump, who warned that the US would cut aid to the region.

Trump's comments look set to make the migrants a campaign issue, just weeks before mid-term elections.

The opposition centre-left Democrats criticized Trump's comments on aid, saying the president did not have the legal right to cut money allocated to Central American nations by Congress.

"President Trump's policy toward Latin America will only make things worse," warned Eliot Engel, the leading Democrat on foreign affairs in the House of Representatives.

Many of the migrants are fleeing economic hardship in Honduras, where about 64 per cent of the population live in poverty, according to official statistics.

The country also had a homicide rate of 43.6 killings per 100,000 people in 2017, one of the highest in the world.