President Trump has laid the blame for the stream of migrants making their way to the US on Central American governments.
President Donald Trump said Monday the United States will start cutting aid to Guatemala, Honduras and El Salvador as a caravan of thousands of mostly Honduran migrants rolled on regardless toward the US border.
The United Nations said more than 7,000 people were now heading toward the US, as more migrants joined the original group, including some Central Americans who were already in Mexico.
Trump meanwhile kept up his almost-daily Twitter attacks on the approaching caravan, calling it a national emergency, and saying he had alerted the US border patrol and military.
"We will now begin cutting off, or substantially reducing, the massive foreign aid" that the United States provides to the three Central American countries, said the president - who has seized upon the crisis in the run-up to US midterm elections, reviving the immigrant-bashing rhetoric that helped get him elected in 2016.
Mexican authorities had managed to block the migrants on the Mexico-Guatemala border after they burst through a series of barriers on the Guatemalan side on Friday. But many later crossed the river below in makeshift rafts before marching north.
The caravan resumed its journey Monday in the state of Chiapas in southern Mexico, setting out from Tapachula, near the border, for the town of Huixtla, around 40 kilometers (25 miles) away.
Many of the migrants had spent the night on the town square or the street, wary of the shelters set up by charities and the government. Moving in a massive ebb and flow, they set out on foot or hitched rides with passing cars and trucks.
"Sadly, it looks like Mexico's Police and Military are unable to stop the Caravan heading to the Southern Border of the United States. Criminals and unknown Middle Easterners are mixed in," Trump said in one tweet.
"I have alerted Border Patrol and Military that this is a National Emergy (sic). Must change laws!"
Activists say the journey of at least 3,000 kilometers (1,800 miles) through Mexico to the US border could take a month.
"We're a little afraid the police could detain us. But if they deport us, we'll just try again," said Noemi Bobadilla, 39, a cleaner from San Pedro Sula, Honduras.
She had already been walking for 13 days, with a friend and her baby.
Rights activists accused Mexico of mistreating the migrants.
Several groups accused the Mexican government in a statement of "arbitrary detentions" and "grave human rights violations" for detaining migrants who tried to enter the country legally and file asylum claims.
Other activists accused Trump of using the caravan to score political points.
"Trump isn't afraid of the caravan, Trump is using the caravan to win Congress," said Irineo Mujica, an activist who helped organize another caravan that also infuriated Trump in April.
US Secretary of State Mike Pompeo, however, denounced the "apparent political motivation of some organizers of the caravan."
About a thousand migrants, including women and children, were still stranded on a border bridge hoping to enter Mexico legally via Guatemala.
Mexican authorities insisted those on the bridge would have to file asylum claims one at a time in order to enter the country.
A separate group of about 1,000 Hondurans started their own march across Guatemala, headed for Mexico and then the United States. The group of men, women and children gathered in Esquipulas before setting out on foot.
The caravan left San Pedro Sula in northern Honduras more than a week ago.
It had comprised between 3,000 and 5,000 people at various times as it moved through Guatemala, according to various sources.
The International Organization for Migration now estimates it comprises just under 7,250 people, said UN deputy spokesman Farhan Haq.
Around 900 migrants - tired of waiting on the bridge - resorted to crossing the Suchiate River below on makeshift rafts and police did not intervene as they clambered up the muddy riverbank on the Mexican side on Saturday.
Many of the migrants are fleeing poverty and insecurity in Honduras, where powerful street gangs rule their turf with brutal violence.
With a homicide rate of 43 per 100,000 citizens, Honduras is one of the most violent countries in the world, according to a Honduran university study.
US aid to Honduras had already been on the decline.
It went from $209.2 billion in 2016 to $181.7 billion last year, according to the Washington Office on Latin America.