CIUDAD ACUNA, Mexico/WASHINGTON, Sept 23 (Reuters) - The U.S. special envoy to Haiti resigned in protest in a letter that blasted the Biden administration for deporting hundreds of migrants back to the crisis-engulfed Caribbean nation from a camp on the U.S.-Mexican border in recent days.

Daniel Foote, a career diplomat named to his post in July, said conditions in Haiti were so bad that U.S. officials were confined to secure compounds. He said the "collapsed state" was unable to support the infusion of returning migrants.

"I will not be associated with the United States' inhumane, counterproductive decision to deport thousands of Haitian refugees and illegal immigrants," Foote said in a letter addressed to Secretary of State Antony Blinken that circulated publicly on Thursday.

Haiti, the poorest nation in the Western Hemisphere, has been badly hit in recent weeks by a presidential assassination, gang violence and a major earthquake.

The United States has returned more than 1,400 migrants from the camp in Del Rio, Texas to Haiti - including families - and moved over 3,200 people for processing away from the encampment, U.S. Department of Homeland Security (DHS) officials said on Thursday. At its peak on Sept. 18 there were some 15,000 people there, around two-thirds of those families, the officials said.

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Many of the migrants say they hope to stay in the United States and seek asylum but because of a widespread expulsion policy in place since the beginning of the coronavirus pandemic which bars most asylum claims, many may not get that opportunity. Some are being loaded onto flights back to Haiti while others are being released into the United States to pursue their immigration cases in court.

Images of a U.S. border agent on horseback using long reins to whip in the direction of migrants trying to cross the Rio Grande caused outrage within the White House and from rights groups and prompted Vice President Kamala Harris on Wednesday to seek answers from Homeland Security Secretary Alejandro Mayorkas.

DHS said patrols on horseback had ceased in Del Rio and the agents involved had been put on administrative duties.

Foote submitted his resignation to Blinken on Wednesday, a State Department spokesperson said, adding that Washington was committed to the long-term well being of Haiti, as well as offering immediate help to returning migrants.

State Department spokesman Ned Price rejected Foote's criticism.

"This is a challenging moment that requires leadership. It is unfortunate that, instead of participating in a solutions-oriented policy process, Special Envoy Foote has both resigned and mischaracterized the circumstances of his resignation."

"He failed to take advantage of ample opportunity to raise concerns about migration during his tenure and chose to resign instead," said Price.

The resignation follows growing pressure on the administration of U.S. President Joe Biden from the United Nations and his fellow Democrats over the treatment of the Haitians.

Biden's handling of what Republicans portray as a crisis at the border - including record numbers of migrant detentions this year - has led to growing disillusionment from advocates who hoped for an end to deterrent measures brought in by his predecessor Donald Trump.

Filippo Grandi, the head of the U.N refugee agency, warned that this week the U.S. expulsions to Haiti might violate international law.

The expulsions have taken place since March 2020 at the beginning of the coronavirus pandemic for public heath reasons. Under the policy known as Title 42, most migrants encountered at the border - largely Central Americans - have been quickly returned to Mexico. Haitians are being expelled back to their home county, however.

At least four deportation flights to Haiti were scheduled for Thursday, according to a flight tracking website and activists who track deportation flights.

Foote's decision to leave was interpreted and welcomed by rights groups as a stern criticism of the administration's immigration strategy.

"A big bold move. And a big deal," said William O'Neill, a lawyer specializing in humanitarian, human rights and refugee law. "He will be missed."

Mexican pressure

In recent days, the population of the squalid encampment under a bridge over the Rio Grande on the border has been reduced to around 4,000 by expulsion flights, detentions and releases.

DHS officials said that determinations on who was expelled and who has been allowed to stay in the United States to pursue their immigration cases were made on a "case-by-case" basis, without providing numbers of releases.

Wade McMullen, an attorney with the Robert F. Kennedy Human Rights organization, said several hundred people, mostly pregnant women and parents with children, had been released in Del Rio over the past several days.

Others have left the dusty riverbank for Mexico to avoid being sent home.

Mexican authorities in Ciudad Acuna, across the border from Del Rio, stepped up security operations on Tuesday with close to 20 patrol cars and numerous officers hoisting heavy-duty firearms lined up along the river where migrants were crossing back and forth between the two countries.

Jean Pie, 48, was awoken shortly after 6 a.m. from where he was sleeping on the grass on the Mexican side as police cars rolled by.

Like many of the Haitians, he had first tried to make a life in South America. Migrants have said they decided to head north when they encountered discrimination and dwindling opportunities in countries like Brazil and Chile.

He said he had spent two months and $3,000 to reach Mexico from Chile, including $500 to a man in Mexico who promised to take him from the southern city of Tapachula near the border with Guatemala to the U.S. border but then disappeared. He was also deported from Mexico to Guatemala by Mexican authorities.

"We are living in constant fear because Mexican migration officials can pick up anyone," he said. "I have run out of money. It is all just too much."

(Reporting by Daina Beth Solomon in Ciudad Acuna and Doina Chiacu in Washington, Additional reporting by Lizbeth Diaz, Mica Rosenberg, Humeyra Pamuk and Kristina Cooke, Editing by Heather Timmons and Alistair Bell)