Images which appear to show border force agents on horseback rounding up migrants who have just crossed a river into the US have sparked an investigation.
The pictures appear to show officers using their reins against the migrants who are attempting to cross the southern border and pushing them back towards the Rio Grande which separates Texas from Mexico.
Homeland Security chief Alejandro Mayorkas said the images "horrified" him and that the department will investigate reports of alleged abuse.
Mr Mayorkas told CNN that “any mistreatment or abuse of a migrant is unacceptable” and added, “the pictures that I’ve observed troubled me profoundly", but would not explicitly say whether the images reflected mistreatment or abuse, deferring instead to a promised investigation.
In recent weeks the border town of Del Rio, Texas, has seen an influx of thousands of migrants from Haiti fleeing an unstable situation in their home country and attempting to enter the US through Mexico.
Around 6,000 Haitian and other migrants have been removed from a makeshift camp in Del Rio, and Mr Mayorkas said there would be a “dramatic change” in the number of migrants there within the next two to four days as it continues the removal process.
The latest increase in migration at the border has led to criticism from Republican groups who claim there is a crisis at the US border. But Mr Mayorkas declined to call the situation a “crisis,” saying only that it’s a “heartbreaking situation” and a “tremendous challenge".
“I by no means diminish the, the humane issue that it presents, but I want to be clear that we do have a plan to address it,” he said.
More than 600 Homeland Security employees, including from the Coast Guard, have been brought to the border town to help in what may be one of the swiftest, large-scale expulsions of migrants and refugees from the United States in decades.
On Monday, speaking from Del Rio, Mr Mayorkas warned Haitians not to come to the US, noting that a temporary extension in protections from deportation for Haitians did not apply to new migrants.
“If you come to the United States illegally, you will be returned. Your journey will not succeed, and you will be endangering your life and your family’s life,” he said.