CIUDAD ACUNA, MEXICO (REUTERS) - More than 10,000 mostly Haitian migrants were bivouacked in a squalid camp under a bridge in southern Texas on Friday (Sept 17), even as hundreds more headed toward the border in a growing humanitarian and political challenge for US President Joe Biden.

The Haitians were joined by Cubans, Venezuelans and Nicaraguans under the Del Rio International Bridge across the Rio Grande connecting Ciudad Acuna in Mexico to Del Rio, Texas.

They slept under light blankets, while a few pitched small tents.

Officials on both sides of the US-Mexico border said the majority of the migrants were Haitians and more are expected to arrive. Most of the people Reuters spoke with had not come direct from Haiti but had made long and harrowing journeys through Mexico and Central and South America.

Several told Reuters they followed routes shared amongst each other on WhatsApp.

In Ciudad Acuna, two workers at the main bus terminal said at least two dozen buses full of Haitians had arrived on Friday, with one putting the estimate of individual arrivals at around 1,100. A Mexican official said the number of migrants was around 12,000 on Friday afternoon.

Reuters saw dozens of mostly Haitian migrants arriving at the terminal, carrying jugs of water, tightly-packed backpacks and folders containing travel documents. Many said they soon planned to head across the shallow Rio Grande.

Ms Ali Sajous, 29, a Haitian migrant, said US officials were handing out numbers for immigration processing and she, her husband and two-year-old daughter who arrived last Monday were No. 910 in the families line. Authorities were processing No. 685 at the time, she said.

Haiti, the poorest nation in the Western Hemisphere, has a troubled history. In July, the president was assassinated, and in August a major earthquake and powerful storm hit the country.

Many of the Haitians interviewed by Reuters said they used to live in South America, often Brazil or Chile, and were headed north now because they could not attain legal status or struggled to secure decent jobs. Some Haitians also said they were encouraged by videos on social media about applying for asylum in the United States.

A Mexican official, speaking on condition of anonymity, said some 500 mostly Haitian migrants took buses on Friday bound for the border city of Reynosa, east of Del Rio and opposite McAllen, Texas. Hundreds of Haitians were already at a border bridge there.

Later on Friday, Mexican news outlet Aristegiu Noticias said migration officials had taken some 400 migrants off the buses in the town of San Fernando. The migrants, who included pregnant women, children and elderly, planned to walk some 150 kilometers to Reynosa, the report said.

Mexico's National Institute of Migration (INM) did not immediately respond to a request for comment.

Hundreds wade through Rio Grande

In the Del Rio area, Reuters witnessed hundreds of migrants wading through the knee-high Rio Grande back into Mexico to stock up on essentials they say they are not receiving on the American side.

Two Haitian migrants said US officials provided a hot meal on Thursday night. Mr Paul Marie-Samise, 32, said he missed it.

The US Border Patrol said on Thursday that it was increasing staffing in Del Rio and providing drinking water, towels and portable toilets. US Customs and Border Protection on Friday announced that because of the influx, it was temporarily and immediately closing its port of entry at Del Rio and re-routing border traffic to Eagle Pass, 57 miles east.

Mr Biden, a Democrat who took office in January, has pledged a more humane approach to immigration than that of former President Donald Trump. The situation in Del Rio is giving ammunition to critics who say Mr Biden's policies have encouraged migrants.

Texas Governor Greg Abbott, a Republican whose administration has arrested migrants for trespassing and is planning to build its own border wall after Mr Biden halted Trump's signature project, signed legislation on Friday providing an additional US$1.8 billion (S$2.43 billion) in state funding for border security over the next two years.

A spokesman for Mr Abbott, Ms Renae Eze, said in an email to Reuters that combined with more than US$1 billion allocated earlier it brought border security funding in the state to the US$3 billion Abbot had pledged in a Twitter post on Thursday.

"We're trying to fix Biden's failure," Mr Abbott tweeted.

US authorities arrested more than 195,000 migrants at the southwest border in August, according to government data released on Wednesday, a slight dip from the previous month but still around 20-year-highs.

While Mr Biden rolled back many of Trump's immigration actions early in his presidency, he left in place a sweeping pandemic-era expulsion policy under which most migrants caught crossing the US-Mexico border are quickly turned back.

The policy issued by the US Centres for Disease Control and Prevention has been criticised by pro-migrant groups and some Democrats as cutting off legal access to asylum. On Thursday, a US federal judge ruled the policy could no longer be applied to families.

The Biden administration on Friday appealed the judge's order, which had been set to take effect in two weeks.

While more than 90 per cent of all single adults caught at the border from Mexico and Central America were quickly expelled under the policy, less than 40 per cent of single adults from other countries were expelled, according to US Customs and Border Protection data from this fiscal year, which began last October.

Many of those who are not expelled hope to be released into the United States so they can apply for asylum, a long process due to a backlogged immigration court system.