CIUDAD ACUÑA, Mexico (AP) — A camp where almost 15,000 migrants had waited  along the Texas border just days ago was dramatically smaller Thursday, while across the river in Mexico, Haitian migrants in a growing camp awoke surrounded by security forces as a helicopter thundered overhead.

As of Thursday, about 4,000 migrants remained under the bridge between Del Rio and Mexico, Department of Homeland Security officials said. The number peaked sharply on Saturday, as migrants driven by confusion over the Biden administration's policies and misinformation on social media converged at the crossing. Food, shelter and medical care was being provided to those who need it officials said.

About 1,400 had been sent to Haiti on 13 flights, rapidly expelled under the pandemic public health authority known as Title 42, DHS officials told reporters. Another 3,200 are in U.S. custody and being processed, while several thousand have returned to Mexico,  DHS officials said, speaking on condition of anonymity to brief journalists about an ongoing operation. 

Mexico's immigration agency had estimated late Wednesday there were as many as 600 migrants in Ciudad Acuña. The riverside camp appeared to hold that many at its peak. Other migrants are scattered through the city in hotels and private homes. A city official said Wednesday that Mexican authorities had removed about 250 Haitian migrants from the city since Sunday evening. Still, "several thousand" migrants returning to Mexico from the Del Rio camp seemed an exaggeration.

The State Department is in talks with Brazil and Chile to allow some Haitians who were previously residing in those countries to return, but the issue is complicated because some no longer have legal status there, the officials said.

The United States and Mexico appeared eager to end the increasingly politicized humanitarian situation at the border, even as the U.S. expulsion of Haitians to their troubled homeland caused  blowback for the administration of President Joe Biden.

The Biden administration's special envoy to Haiti,  Daniel Foote, submitted a letter of resignation  protesting the "inhumane" large-scale expulsions of Haitian migrants, U.S. officials said Thursday.

In Mexico, migrants who had camped in a park beside the river in Ciudad Acuña found state police trucks spaced every 30 feet or so between their tents and the water's edge. Still, after anxious minutes of indecision, dozens of families opted to hustle into the river and cross at a point where there was only one municipal police vehicle, calculating it was better to take their chances with U.S. authorities.

The entrance to the park was blocked and just outside, National Guard troops and immigration agents waited along with three buses. A helicopter flew overhead.

The camp's usual early morning hum was silenced as migrants tried to decide what to do.

Guileme Paterson, a 36-year-old from Haiti, appeared dazed. "It is a difficult moment," she said before beginning to cross the Rio Grande with her husband and their four children.

The Mexican authorities' operation appeared designed to drive the migrants back across the river into Texas. A fence line and the line of state police vehicles funneled the migrants back to the crossing point they had been using all week.

The buses that had been waiting left empty. The majority of the camp's migrants remained.

"Bad, bad, bad, things are going badly," said Michou Petion, carrying her 2-year-old son in her arms toward the river. Her husband carried bags of their belongings and several pairs of sneakers dangled around his neck.

"The U.S. is deporting a lot to Haiti, now I don't know if I can enter or leave," Petion said.

"We're talking to a lot of people and they are nervous, they're afraid, they're desperate," said Christoph Jankhoefer of the humanitarian organization Doctors Without Borders, which is working in the Ciudad Acuña camp. "Two women were crying because they didn't want to be deported to Guatemala."

In recent weeks, Mexican authorities had been dropping off migrants from other countries at the Guatemalan border. 

Some Haitians are being allowed to remain in the U.S. at least temporarily to seek asylum or to stay under some other claim to residency, with notices to appear later before immigration authorities. DHS officials declined to specify the number but said they are people with particular "vulnerabilities," which can mean they have young children or are pregnant, or because the U.S. doesn't have capacity to hold them in detention, especially during the pandemic.

There are no plans to stop expelling other migrants under Title 42, despite pressure from Democratic lawmakers, who say migrants are being sent back to a troubled country that some left more than a decade ago.

The provision was put in place by the Trump administration in March 2020 to justify restrictive immigration policies in an effort to prevent the spread of the coronavirus. But the Biden administration has used Title 42 to justify the deportation of Haitian migrants. 

A federal judge late last week ruled the regulation was improper and gave the government two weeks before its use was to be halted, but the Biden administration on Monday appealed the decision. 

UNICEF said in a statement Thursday that based on initial estimates, more than two out of three migrants expelled to Haiti are women and children, including newborns, and about 40% in Del Rio are children.

"Haiti is reeling from the triple tragedy of natural disasters, gang violence and the COVID-19 pandemic," said Henrietta Fore, UNICEF's executive director, who said those sent back without adequate protection "find themselves even more vulnerable to violence, poverty and displacement — factors that drove them to migrate in the first place."

DHS officials said about two-thirds of the people under the bridge are families and the rest are single adults. There have been relatively few unaccompanied children. 

The DHS said Thursday that the Border Patrol has temporarily stopped using horses, following outcry over images and video of agents on horseback appearing to use aggressive tactics against migrants.

Just outside the camp, a line of Border Patrol agents and Texas state troopers combed through the tall carrizo cane Thursday morning, apparently looking for any migrants who were on the outskirts.

Meanwhile, Foote, who was appointed as U.S. envoy for Haiti only in July, wrote Secretary of State Antony Blinken that he was stepping down immediately "with deep disappointment and apologies to those seeking crucial changes."

"I will not be associated with the United States inhumane, counterproductive decision to deport thousands of Haitian refugees and illegal immigrants to Haiti, a country where American officials are confined to secure compounds because of the danger posed by armed gangs to daily life," he wrote. "Our policy approach to Haiti remains deeply flawed, and my policy recommendations have been ignored and dismissed, when not edited to project a narrative different from my own." 

The career diplomat was known to be deeply frustrated with what he considered a lack of urgency in Washington and a glacial pace on efforts to improve conditions in Haiti.

State Department spokesman Ned Price disputed Foote's assertions, saying his proposals had been "fully considered in a rigorous and transparent policy process." 

"Some of those proposals were determined to be harmful to our commitment to the promotion of democracy in Haiti and were rejected during the policy process. For him to say his proposals were ignored is simply false," Price said.

At least one top official in Haiti cheered Foote's resignation as he accused the U.S. Border Patrol of violating the rights of Haitian migrants.

"This is the first time we see a U.S. diplomat who has decided to go against the will of the U.S. government," Mathias Pierre, Haiti's elections minister, told The Associated Press. "We salute that."

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Lozano reported from Del Rio, Texas. AP journalists Julio Cortez in Del Rio, Texas, Joshua Goodman in Miami, Matthew Lee in New York, Danica Coto in San Juan, Puerto Rico, contributed to this story.